i386: PR target/113231: Improved costs in Scalar-To-Vector (STV) pass.

This patch improves the cost/gain calculation used during the i386 backend's
SImode/DImode scalar-to-vector (STV) conversion pass.  The current code
handles loads and stores, but doesn't consider that converting other
scalar operations with a memory destination, requires an explicit load
before and an explicit store after the vector equivalent.

To ease the review, the significant change looks like:

         /* For operations on memory operands, include the overhead
            of explicit load and store instructions.  */
         if (MEM_P (dst))
           igain += !optimize_insn_for_size_p ()
                    ? -COSTS_N_BYTES (8);
                    : (m * (ix86_cost->int_load[2]
                            + ix86_cost->int_store[2])
                       - (ix86_cost->sse_load[sse_cost_idx] +
                          ix86_cost->sse_store[sse_cost_idx]));

however the patch itself is complicated by a change in indentation
which leads to a number of lines with only whitespace changes.
For architectures where integer load/store costs are the same as
vector load/store costs, there should be no change without -Os/-Oz.

2024-01-07  Roger Sayle  <roger@nextmovesoftware.com>
	    Uros Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	PR target/113231
	* config/i386/i386-features.cc (compute_convert_gain): Include
	the overhead of explicit load and store (movd) instructions when
	converting non-store scalar operations with memory destinations.
	Various indentation whitespace fixes.

gcc/testsuite/ChangeLog
	PR target/113231
	* gcc.target/i386/pr113231.c: New test case.