Introduce API for codegen getting temp registers from gtRsvdRegs

LSRA puts a set of temporary registers needed by a node in the
gtRsvdRegs register mask. Currently, evey codegen function that
needs to use a temporary register manually manipulates this mask.

Introduce a few functions to make this simpler and more regular:

1. GetSingleTempReg() // Gets a temp; asserts there is exactly one temp reg.
2. ExtractTempReg() // Gets the lowest temp, removes it from gtRsvdRegs so subsequent calls don't see it.
3. AvailableTempRegCount() // Returns a count of available temp registers.

All take an optional register mask, so you can extract a register from
just the set specified by the mask (typically RBM_ALLINT or RBM_ALLFLOAT).

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