cmd/compile: constant fold SSA bool to int conversions
Shaves off a few instructions here and there.
file before after Δ %
go/types.s 322118 321851 -267 -0.083%
go/internal/gcimporter.s 34937 34909 -28 -0.080%
go/internal/gccgoimporter.s 56493 56474 -19 -0.034%
cmd/compile/internal/ssa.s
3926994 3927177 +183 +0.005%
total
18862670 18862539 -131 -0.001%
Change-Id: I724f32317b946b5138224808f85709d9c097a247
Reviewed-on: https://go-review.googlesource.com/c/go/+/221428
Reviewed-by: Keith Randall <khr@golang.org>