cmd/compile: avoid extending when already sufficiently shifted on loong64
This reduces 744 instructions from the go toolchain binary on loong64.
file before after Δ %
asm 599282 599222 -60 -0.0100%
cgo 513606 513534 -72 -0.0140%
compile
2939250 2939146 -104 -0.0035%
cover 564136 564056 -80 -0.0142%
fix 895622 895546 -76 -0.0085%
link 759460 759376 -84 -0.0111%
preprofile 264960 264916 -44 -0.0166%
vet 869964 869888 -76 -0.0087%
go
1712990 1712890 -100 -0.0058%
gofmt 346416 346368 -48 -0.0139%
total
9465686 9464942 -744 -0.0079%
Change-Id: I32dfa7506d0458ca0b6de83b030c330cd2b82176
Reviewed-on: https://go-review.googlesource.com/c/go/+/725720
Auto-Submit: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Keith Randall <khr@google.com>