PR target/116275: Handle STV of *extenddi2_doubleword_highpart on i386.

PR target/116275: Handle STV of *extenddi2_doubleword_highpart on i386.

This patch resolves PR target/116275, a recent ICE-on-valid regression on
-m32 caused by my recent change to enable STV of DImode arithmeric right
shift on non-AVX512VL targets.  The oversight is that the i386 backend
contains an *extenddi2_doubleword_highpart instruction (whose pattern
is an arithmetic right shift of a left shift) that optimizes the case where
sign-extension need only update the highpart word of a DImode value when
generating 32-bit code (!TARGET_64BIT).  STV accepts this pattern as a
candidate, as there are patterns to handle this form of extension on SSE
using AVX512VL instructions (and previously ASHIFTRT was only allowed on
AVX512VL).  Now that ASHIFTRT is a candidate on non-AVX512vL targets, we
either need to check that the first operand is a register, or as done
below provide the define_insn_and_split that provides a non-AVX512VL
implementation of *extendv2di_highpart_stv.

The new testcase only ICEed with -m32, so this test could be limited to
target ia32, but there's no harm also running this test on -m64 to
provide a little extra test coverage.

2024-08-12  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR target/116275
	* config/i386/i386.md (*extendv2di2_highpart_stv_noavx512vl): New
	define_insn_and_split to handle the STV conversion of the DImode
	pattern *extendsi2_doubleword_highpart.

gcc/testsuite/ChangeLog
	PR target/116275
	* g++.target/i386/pr116275.C: New test case.