cmd/compile/internal/x86: fix zero-range merging logic
All other architectures merge stack-zeroing ranges if there are at
most two pointers/registers of memory between them, but x86 is
erroneously coded to require *exactly* two.
Shaves a tiny amount of text size off cmd/go when building for
GOARCH=386 and eliminates an unnecessary inconsistency between x86's
defframe and the other GOARCHes'.
text data bss dec hex filename
5241015 191051 93336
5525402 544f9a go.before
5240224 191051 93336
5524611 544c83 go.after
Change-Id: Ib15ec8c07bca11e824640f0ab32abfc4bb160496
Reviewed-on: https://go-review.googlesource.com/30050
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>