cmd/6g: generate boolean values without jumps

cmd/6g: generate boolean values without jumps

[Depends on issue 8545 being fixed correctly; in the interim, use CL 125510043.]

5g and 8g (and 9g?) will be handled in subsequent CLs.

Top movers in stdlib benchmarks:

benchmark                                    old ns/op      new ns/op      delta
BenchmarkNextafter64                         7.72           5.86           -24.09%
BenchmarkSignbit                             2.13           2.78           +30.52%
BenchmarkNextafter32                         7.98           6.17           -22.68%
BenchmarkClientServer                        110304         86169          -21.88%
BenchmarkContendedSemaphore                  72.0           86.1           +19.58%
BenchmarkIndexByte32                         6.94           8.28           +19.31%
BenchmarkIndexRuneFastPath                   27.3           22.9           -16.12%
BenchmarkFinalizer                           939372         1092025        +16.25%
BenchmarkTCP4OneShotTimeout                  1123770        969409         -13.74%
BenchmarkUnquoteEasy                         190            217            +14.21%
BenchmarkCompareBytesToNil                   6.92           6.13           -11.42%
BenchmarkComplex128DivNisNaN                 11.4           10.1           -11.40%
BenchmarkEqual16                             7.18           6.38           -11.14%
BenchmarkEqual9                              7.18           6.38           -11.14%
BenchmarkEqual15                             7.18           6.41           -10.72%
BenchmarkSearchWrappers                      162            145            -10.49%
BenchmarkEqual20                             8.01           7.18           -10.36%
BenchmarkCompareBytesBigIdentical            5.32           4.79           -9.96%
BenchmarkInterfaceSmall                      15.2           13.8           -9.21%
BenchmarkEqual32                             8.81           8.00           -9.19%
BenchmarkTrimSpace                           52.7           58.0           +10.06%
BenchmarkCompareBytesEmpty                   5.87           5.34           -9.03%
BenchmarkMapStringKeysEight_64               21.8           23.9           +9.63%
BenchmarkMapStringKeysEight_16               22.8           20.8           -8.77%
BenchmarkMapStringKeysEight_1M               21.9           24.0           +9.59%
BenchmarkMapStringKeysEight_32               21.9           24.0           +9.59%
BenchmarkCompareBytesIdentical               5.85           5.34           -8.72%
BenchmarkAcosh                               32.2           35.2           +9.32%
BenchmarkMin                                 3.20           2.93           -8.44%
BenchmarkIPString                            2120           1954           -7.83%
BenchmarkChanSem                             61.9           57.3           -7.43%
BenchmarkCompareBytesDifferentLength         6.91           6.40           -7.38%
BenchmarkCompareBytesEqual                   6.91           6.40           -7.38%
BenchmarkCompareBytesSameLength              6.92           6.41           -7.37%

All regressions I've investigated are due to incidental code movement, generally due to generated code being shorter.

Full benchcmp output is at https://gist.github.com/josharian/e49e89916101c4f145fc.

The godoc binary is ~0.2% smaller after this CL.

Update issue 5729