cmd/compile,testing: implement one-time rampup logic for testing.B.Loop

cmd/compile,testing: implement one-time rampup logic for testing.B.Loop

testing.B.Loop now does its own loop scheduling without interaction with b.N.
b.N will be updated to the actual iterations b.Loop controls when b.Loop returns false.

This CL also added tests for fixed iteration count (benchtime=100x case).

This CL also ensured that b.Loop() is inlined.

For #61515

Change-Id: Ia15f4462f4830ef4ec51327520ff59910eb4bb58
Reviewed-on: https://go-review.googlesource.com/c/go/+/627755
Reviewed-by: Michael Pratt <mpratt@google.com>
Commit-Queue: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>