cmd/compile: initialize loop depths

cmd/compile: initialize loop depths

Regalloc uses loop depths - make sure they are initialized!

Test to make sure we aren't pushing spills into loops.

This fixes a generated-code performance bug introduced with
the better spill placement change:
https://go-review.googlesource.com/c/34822/

Update #19595

Change-Id: Ib9f0da6fb588503518847d7aab51e569fd3fa61e
Reviewed-on: https://go-review.googlesource.com/38434
Reviewed-by: David Chase <drchase@google.com>