[optimizing] More x86_64 code improvements

Use the constant area some more, use 32-bit immediates in movq
instructions when possible, and other small tweaks.

Remove the commented out code for Math.Abs(float/double) as it would
fail for baseline compiler due to the output being the same as the
input.

Change-Id: Ifa39f1865b94cec2e1c0a99af3066a645e9d3618
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>