Compiling

func F() {
	var d int8
	d >>= 1000
}

with x/tools/go/ssa produces:

func F():
0:                                                                entry P:0 S:0
	t0 = 0:int8 >> 1000:int8                                           int8
	return

See https://go.dev/play/p/XugtNClNWNp?v=gotip

But a 1000:int8 constant is ill typed, because 1000 is out of range for int8. Moreover, there's no reason 1000 needs to be int8, it's fine as uint.

Notably, if the code is changed to d = d >> 1000, then the 1000 constant is correctly typed as uint.

I think the fix is in builder.assignOp: it should only apply b.emitConv(val, oldv.Type()) when op isn't LSH or RSH. When it is a shift, it should just use val as is.

/cc @timothy-king @zpavlinovic