cmd/compile: missed opportunity to coalesce loads that compare with constants

[josharian]

func f(x *[16]byte) bool {
	return x[0] == 0 && x[1] == 0 &&
		x[2] == 0 && x[3] == 0 &&
		x[4] == 0 && x[5] == 0 &&
		x[6] == 0 && x[7] == 0 &&
		x[8] == 0 && x[9] == 0 &&
		x[10] == 0xff && x[11] == 0xff
}

This could compile on amd64 to something like MOVQ to load the first 8 bytes, TESTQ to check whether the first 8 bytes are zero, MOVL to load the next 4 bytes, CMPL to test those bytes. Instead, we currently generate a series of 12 CMPBs.

Low priority.