Auto-generate register masks and load them through Config.

Passed toolstash -cmp on AMD64.

Tests phi_ssa.go and regalloc_ssa.go in cmd/compile/internal/gc/testdata
passed on ARM.

Updates #15365.

Change-Id: I393924d68067f2dbb13dab82e569fb452c986593
Reviewed-on: https://go-review.googlesource.com/23292Reviewed-by: David Chase <drchase@google.com>