The register-caching fast path reduced an already-cached wider value to
a byte/halfword with EXTSB/EXTSH (sign extend). OP_LOAD1/OP_LOAD2 are
unsigned loads, so any cached byte >= 0x80 (or halfword >= 0x8000) came
back negative, diverging from the interpreter and the fresh-load path
(LBZ/LHZ). This corrupted byte-sized cgame values and made HUD/item
icons disappear under the ppc64le JIT (CPMA, some OpenArena mods).

Use CLRLDI to zero-extend, matching reduce_map_size() unsigned constant
folding and the aarch64 JIT UXTB/UXTH. The genuine OP_SEX8/OP_SEX16
sign-extends are unaffected.

Fixes ec-/Quake3e#411

…load

The cached OP_LOAD1/LOAD2 fast path zero-extended the cached register in
place. If that register was still a live value elsewhere on the opStack
(e.g. a wider constant reused across stores of different sizes, or a
prior load result still pending), the in-place reduction corrupted that
value - the JIT returned a result differing from the interpreter by a
power of two.

Mirror the clone-on-alias guard already used by load_rx_opstack/finish_rx:
when search_opstack() shows the register is live elsewhere, zero-extend a
copy and leave the cached register untouched; otherwise reduce in place
as before. OP_LOAD4 is unchanged.

Found and verified with a JIT-vs-interpreter differential fuzzer:
reproduced at iteration 17966 before, 0 divergences over 5,000,000
iterations after.