i386: Fine tune STV register conversion costs for -Os.

The eagle-eyed may have spotted that my recent testcases for DImode shifts
on x86_64 included -mno-stv in the dg-options.  This is because the
Scalar-To-Vector (STV) pass currently transforms these shifts to use
SSE vector operations, producing larger code even with -Os.  The issue
is that the compute_convert_gain currently underestimates the size of
instructions required for interunit moves, which is corrected with the
patch below.

For the simple test case:

unsigned long long shl1(unsigned long long x) { return x << 1; }

without this patch, GCC -m32 -Os -mavx2 currently generates:

shl1:	push   %ebp		 // 1 byte
	mov    %esp,%ebp	 // 2 bytes
	vmovq  0x8(%ebp),%xmm0	 // 5 bytes
	pop    %ebp		 // 1 byte
	vpaddq %xmm0,%xmm0,%xmm0 // 4 bytes
	vmovd  %xmm0,%eax	 // 4 bytes
	vpextrd $0x1,%xmm0,%edx  // 6 bytes
	ret			 // 1 byte  = 24 bytes total

with this patch, we now generate the shorter

shl1:	push   %ebp		// 1 byte
	mov    %esp,%ebp	// 2 bytes
	mov    0x8(%ebp),%eax	// 3 bytes
	mov    0xc(%ebp),%edx	// 3 bytes
	pop    %ebp		// 1 byte
	add    %eax,%eax	// 2 bytes
	adc    %edx,%edx	// 2 bytes
	ret			// 1 byte  = 15 bytes total

Benchmarking using CSiBE, shows that this patch saves 1361 bytes
when compiling with -m32 -Os, and saves 172 bytes when compiling
with -Os.

2023-10-24  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/i386/i386-features.cc (compute_convert_gain): Provide
	more accurate values (sizes) for inter-unit moves with -Os.