i386: Improve split of *extendv2di2_highpart_stv_noavx512vl.

This patch follows up on the previous patch to fix PR target/116275 by
improving the code STV (ultimately) generates for highpart sign extensions
like (x<<8)>>8.  The arithmetic right shift is able to take advantage of
the available common subexpressions from the preceding left shift.

Hence previously with -O2 -m32 -mavx -mno-avx512vl we'd generate:

        vpsllq  $8, %xmm0, %xmm0
        vpsrad  $8, %xmm0, %xmm1
        vpsrlq  $8, %xmm0, %xmm0
        vpblendw        $51, %xmm0, %xmm1, %xmm0

But with improved splitting, we now generate three instructions:

        vpslld  $8, %xmm1, %xmm0
        vpsrad  $8, %xmm0, %xmm0
        vpblendw        $51, %xmm1, %xmm0, %xmm0

This patch also implements Uros' suggestion that the pre-reload
splitter could introduced a new pseudo to hold the intermediate
to potentially help reload with register allocation, which applies
when not performing the above optimization, i.e. on TARGET_XOP.

2024-08-15  Roger Sayle  <roger@nextmovesoftware.com>
	    Uros Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog
	* config/i386/i386.md (*extendv2di2_highpart_stv_noavx512vl): Split
	to an improved implementation on !TARGET_XOP.  On TARGET_XOP, use
	a new pseudo for the intermediate to simplify register allocation.

gcc/testsuite/ChangeLog
	* g++.target/i386/pr116275-2.C: New test case.

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