Most 64-bit x86 ops can only take a signed 32-bit constant.
Clean up our rewrite rules to enforce this restriction.

Modify the assembler to fail if the offset does not fit
in the instruction.

That last check triggers a few times on weird testing code.
Suppress those errors if the compiler itself generated errors.

Fixes #14862

Change-Id: I76559af035b38483b1e59621a8029fc66b3a5d1e
Reviewed-on: https://go-review.googlesource.com/20815Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>