[ARM64] Only extend if src size is less than target

[yrp604]

Shink expr tree sizes with equivalent semantics. Developed on top of #7296 for my convenience.

This changes 3ish instructions:

• ldr variants -- conditionally emit the zero/sign extension when needed.
• fmov variants -- conditionally emit the zero extension when needed. I don't think there is existing test coverage for the ENC_FMOV_64VX_FLOAT2INT case, but the comment in decode_scratchpatd indicates it's only used with 64 bit registers.
• ubfx/ubfiz -- there are no encodings where the second argument is a different size than the first.

This removes about 700 total extensions from an arm64 ntdll.

Also added this check to the liftcheck wishlist in #2612

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