cmd/compile: add loop invariant code motion

(I copied Ilya Tocar's CL 27656 and heavily modified it.)

This adds an optimization that moves loop invariant computations
out of the loop. For example:

a:= ...
for ... {
b:= a + 15
  // uses of b
}

Turns into

a:= ...
b:= a + 15
for ... {
  // uses of b
}

Change-Id: I36c8c7e2b3bc1c5e6b4b293bed3a76dc20d6c825
Reviewed-on: https://go-review.googlesource.com/c/go/+/697235
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>

0 files changed, 0 insertions, 0 deletions