Emit IceVariable stack references using proper Intel syntax.

This should actually be done in the IceTargetLowering class.
Use the right Intel syntax for a pseudo-push of an xmm register.
Formatting fixes.
Register allocation fixes.

1. For precolored variables, initialize their RegNumTmp field to their
precolored RegNum so that the code that searches the Unhandled list
for precolored registers properly removes their registers from
consideration.

2. Make sure we are actually masking the set of registers under
consideration according to the type of the variable.