Added direct memory operand support for comiss/sd on x86.

Rationale:
Memory operands allows for shorter instruction sequences.
Something that may be used e.g. the upcoming implementation
of the round intrinsic.

Bug=26327751

Change-Id: Ifd976ed86917de1879fa036cbbbff5edf4fb741d
Test: assembler_x86_test
3 files changed