compiler: adjust code to avoid shadowing local variables

Also add a couple of missing calls to free after mpz_get_str.

This should make the code clean with respect to -Wshadow=local.

Based on patch by Bernd Edlinger.

Change-Id: Iec12a8b75ccf51ba4c6593c25d1a5dc9647d3afd
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/198837
Reviewed-by: Than McIntosh <thanm@google.com>
7 files changed