PR target/108229: A minor STV compute_convert_gain tweak on x86.

PR target/108229: A minor STV compute_convert_gain tweak on x86.

This patch addresses PR target/108229, which is a change in code
generation during the STV pass, due to the recently approved patch
to handle vec_select (reductions) in the vector unit.  The recent
change is innocent, but exposes a latent STV "gain" calculation issue
that is benign (or closely balanced) on most microarchitectures.

The issue is when STV considers converting PLUS with a MEM operand.

On TARGET_64BIT (m=1):
	addq 24(%rdi), %rdx		// 4 bytes
or with -m32 (m=2)
        addl    24(%esi), %eax		// 3 bytes
        adcl    28(%esi), %edx		// 3 bytes
is being converted by STV to
        vmovq   24(%rdi), %xmm5		// 5 bytes
        vpaddq  %xmm5, %xmm4, %xmm4	// 4 bytes

The current code in general_scalar_chain::compute_convert_gain
considers that scalar unit addition is replaced with a vector
unit addition (usually about the same cost), but doesn't consider
anything special about MEM operands, assuming that a scalar load
gains/costs nothing compared to a vector load.  We can allow the
backend slightly better fine tuning by including in the gain
calculation that m scalar loads are being replaced by one vector
load, and when optimizing for size including that we're increasing
code size (e.g. an extra vmovq instruction for a MEM operand).

This patch is a win on the CSiBE benchmark when compiled with -Os.

2023-01-03  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/108229
	* config/i386/i386-features.cc
	(general_scalar_chain::compute_convert_gain) <case PLUS>: Consider
	the gain/cost of converting a MEM operand.