Please answer these questions before submitting your issue. Thanks!

1. What version of Go are you using (go version)?
   tip + patch https://go-review.googlesource.com/#/c/20833/3
2. What operating system and processor architecture are you using (go env)?
   linux + amd64
3. What did you do?
   Compile

package main

var si []int = make([]int, 10)

var X interface{}

func use(y interface{}) {
        X = y
}

func main() {
        const ci64big int64 = 1 << 31
        use(si[ci64big])
}

1. What did you expect to see?
   To compile.
2. What did you see instead?
   ./foo.go:15: invalid instruction: 00054 (/tmp/foo.go:13) ADDQ $17179869184, CX
3. Additional information
   The manual http://support.amd.com/TechDocs/24594.pdf page 79 says that ADDQ imm only works with AX, but here CX is generated.

I believe the bug is here:
{name: "ADDQconst", argLength: 1, reg: gp11, asm: "ADDQ", aux: "Int64", resultInArg0: true, typ: "UInt64"}, // arg0 + auxint
or in the actual encoding of the instruction. We should probably fallback to LEAQ if the output goes to a different register.

@randall77