YJIT: use shorter encoding for mov(r64,imm) when unambiguous (#5081)

* YJIT: use shorter encoding for mov(r64,imm) when unambiguous

Previously, for small constants such as `mov(RAX, imm_opnd(Qundef))`,
we emit an instruction with an 8-byte immediate. This form commonly
gets the `movabs` mnemonic.

In 64-bit mode, 32-bit operands get zero extended to 64-bit to fill the
register, so when the immediate is small enough, we can save 4 bytes by
using the `mov` variant that takes a 32-bit immediate and does a zero
extension.

Not implement with this change, there is an imm32 variant of `mov` that
does sign extension we could use. When the constant is negative, we
fallback to the `movabs` form.

In railsbench, this change yields roughly a 12% code size reduction for
the outlined block.

Co-authored-by: Jemma Issroff <jemmaissroff@gmail.com>

* [ci skip] comment edit. Please squash.

Co-authored-by: Jemma Issroff <jemmaissroff@gmail.com>

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