x86 has a lot of instructions that require the output to be in the same
register as one of the inputs.  When allocating the output register,
allocate the same register as the input if it is available.

Improves the performance of golang.org/x/crypto/sha3 by
10% (from 6% slower than 1.6 to 4% faster).

Fixes #14745

Change-Id: I4d81785240c9368e4dc75107b45c959d200df8e6
Reviewed-on: https://go-review.googlesource.com/20488Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>