[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.

1. revisit the temporary changes in IsExtendedGPReg, currently it simply return false in any cases, we will need to add correct EGPR checks to it.
2. 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.
3. (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.