Reg allocator skips flag-typed values. Flag allocator uses the type
and whether the op has "clobberFlags" set.

Tested on AMD64, ARM, ARM64, 386. Passed 'toolstash -cmp' on AMD64.
PPC64 is coded blindly.

Change-Id: Ib1cc27efecef6a1bb27f7d7ed035a582660d244f
Reviewed-on: https://go-review.googlesource.com/25480
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>