internal-fn: Reuse SUBREG_PROMOTED_VAR_P handling

internal-fn: Reuse SUBREG_PROMOTED_VAR_P handling

expand_fn_using_insn has code to handle SUBREG_PROMOTED_VAR_P
destinations.  Specifically, for:

  (subreg/v:M1 (reg:M2 R) ...)

it creates a new temporary register T, uses it for the output
operand, then sign- or zero-extends the M1 lowpart of T to M2,
storing the result in R.

This patch splits this handling out into helper routines and
uses them for other instances of:

  if (!rtx_equal_p (target, ops[0].value))
    emit_move_insn (target, ops[0].value);

It's quite probable that this doesn't help any of the other cases;
in particular, it shouldn't affect vectors.  But I think it could
be useful for the CRC work.

gcc/
	* internal-fn.cc (create_call_lhs_operand, assign_call_lhs): New
	functions, split out from...
	(expand_fn_using_insn): ...here.
	(expand_load_lanes_optab_fn): Use them.
	(expand_GOMP_SIMT_ENTER_ALLOC): Likewise.
	(expand_GOMP_SIMT_LAST_LANE): Likewise.
	(expand_GOMP_SIMT_ORDERED_PRED): Likewise.
	(expand_GOMP_SIMT_VOTE_ANY): Likewise.
	(expand_GOMP_SIMT_XCHG_BFLY): Likewise.
	(expand_GOMP_SIMT_XCHG_IDX): Likewise.
	(expand_partial_load_optab_fn): Likewise.
	(expand_vec_cond_optab_fn): Likewise.
	(expand_vec_cond_mask_optab_fn): Likewise.
	(expand_RAWMEMCHR): Likewise.
	(expand_gather_load_optab_fn): Likewise.
	(expand_while_optab_fn): Likewise.
	(expand_SPACESHIP): Likewise.