JIT: Avoid internal register allocation when possible for PUTARG_SPLIT (#76443)

JIT: Avoid internal register allocation when possible for PUTARG_SPLIT (#76443)

We can almost always avoid allocating an internal register, which may be
expensive if LR is picked since we cannot use thumb encoding for it.

The only case where we need an internal register is when we have a
source that is in a register, and we have a single taget register to
place that conflicts with that source register. The to-stack copies then
need an intermediate scratch register to avoid clobbering the source
register.