This rule is meant for code optimization, but it makes other rules
potentially more complex, as they need to cope with the fact that
a 32-bit op (BTLconst) can appear everywhere a 64-bit rule maches.

Move the optimization to opcode expansion instead. Tests will be
added in following CL.

Change-Id: Ica5ef291e7963c4af17c124d4a2869e6c8f7b0c7
Reviewed-on: https://go-review.googlesource.com/99995Reviewed-by: Keith Randall <khr@golang.org>