When instructions add, and, or, xor, and movd have
constant operands in some cases more instructions are
generated than necessary by the assembler.

This adds more opcode/operand combinations to the optab
and improves the code generation for the cases where the
size and sign of the constant allows the use of 1
instructions instead of 2.

Example of previous code:
	oris r3, r0, 0
	ori  r3, r3, 65533

now:
	ori r3, r0, 65533

This does not significantly reduce the overall binary size
because the improvement depends on the constant value.
Some procedures show a 1-2% reduction in size. This improvement
could also be significant in cases where the extra instructions
occur in a critical loop.

Testcase ppc64enc.s was added to cmd/asm/internal/asm/testdata
with the variations affected by this change.

Updates #23845

Change-Id: I7fdf2320c95815d99f2755ba77d0c6921cd7fad7
Reviewed-on: https://go-review.googlesource.com/95135
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>