differentiate between 64 bit and 128 bit (R_SUBMMWHOLE) SSE vector regs,
    and support spilling/assembling for R_SUBQ SSE registers (8 bytes)
    (mantis #23962)

    We currently never use the full 128 bit of an SSE register, and
    spilling for those hasn't been implemented yet either (R_SUBMMWHOLE
    SSE regs are spilled into a 4-byte temp currently -> can overwrite data)

git-svn-id: trunk@23700 -