i965/fs: Skip emitting MACH/MOV for small integers.

i965/fs: Skip emitting MACH/MOV for small integers.

The vector backend already implemented this optimization, but
surprisingly, we never bothered to implement it in the scalar backend.

In addition to saving two instructions, this eliminates a use of the
accumulator as an explicit source, which is unsupported in SIMD16 mode
on Gen7+, which could help us gain SIMD16 programs.

Cuts 19.23% of the instructions in dolphin/efb2ram.shader_test.

v2: Rebase on is_16bit_integer_constant -> is_uint16_constant rename.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>