cmd/compile: keep variables alive in testing.B.Loop loops

cmd/compile: keep variables alive in testing.B.Loop loops

For the loop body guarded by testing.B.Loop, we disable function inlining and devirtualization inside. The only legal form to be matched is `for b.Loop() {...}`.

For #61515

Change-Id: I2e226f08cb4614667cbded498a7821dffe3f72d8
Reviewed-on: https://go-review.googlesource.com/c/go/+/612043
Reviewed-by: Michael Pratt <mpratt@google.com>
TryBot-Bypass: Junyang Shao <shaojunyang@google.com>
Commit-Queue: Junyang Shao <shaojunyang@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>