cmd/compile: move likely used values into registers before entering loop
When we are about to enter a loop, we try to put values that will
be used soon into registers, so we put them into registers once outside
the loop instead of N times inside the loop.
But we currently don't kick out values we won't use for a while to
make room. This CL does that. So even if register pressure is large
around the loop, we can still use registers for all the in-loop
values. The values generating the register pressure, but not used
inside the loop, will get spilled around the loop.
This is particularly useful for Phi arguments. We want to promote
from fixed zero registers or other rematerializeable values to
a general register before the loop starts.
Fixes #77299
Change-Id: I00efc3d3014163aaf377693830c7d7957eaa515f
Reviewed-on: https://go-review.googlesource.com/c/go/+/743640
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Keith Randall <khr@google.com> Reviewed-by: Junyang Shao <shaojunyang@google.com>