[APX-REX2] Left-over TODOs after REX2 encoding changes

[Ruihan-Yin]

With #106557, we provided the REX2 encoding functionality to JIT backend, but due to the definition for extended GP registers (REG_R16~REG_R31) is still missing, I'm opening this issue to track TODOs needed to complete when we have register allocator properly updated.

• revisit the temporary changes in IsExtendedGPReg, currently it simply return false in any cases, we will need to add correct EGPR checks to it.

• potentially, we will need to revisit the REX2 stress logic when REX2 and register allocator changes are all there, make sure the encoding stress mode is working properly.

• (tracked by separate issue: [X86] add missing TP and latency information for rcl_N/rcr_N #110415): add missing tp and latency information to rcl_N/rcr_N, and enable them for REX2.

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[JulieLeeMSFT]

Assigned to @Ruihan-Yin for now, but we can redistribute as needed.
CC @BruceForstall, @tannergooding, and @anthonycanino.