cmd/gc: eliminate some pointer arithmetic
In mparith, all the a1-- are problematic. Rewrite it all without pointers.
It's clearer anyway.
In popt, v is problematic because it is used both as a fixed pointer
(v = byvar[i]) and as a moving pointer (v = var; v++) aka slice.
Eliminate pointer movement.
Tested that this still produces bit-for-bit output for 'go build -a std'
compared to
d260756 (current master).
Change-Id: I1a1bed0f98b594c3864fe95075dd95f9b52113e0
Reviewed-on: https://go-review.googlesource.com/4645
Reviewed-by: Austin Clements <austin@google.com>