[optimizing] Tune some x86_64 moves

Generate Moves of constant FP values by loading from the constant table.

Use 'movl' to load a 64 bit register for positive 32-bit values, saving
a byte in the generated code by taking advantage of the implicit
zero extension.

Change a couple of xorq(reg, reg) to xorl to (potentially) save a byte
of code per xor.

Change-Id: I5b2a807f0d3b29294fd4e7b8ef6d654491fa0b01
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>