PR rtl-optimization/123585: ICE in vec_select simplification on x86_64.

PR rtl-optimization/123585: ICE in vec_select simplification on x86_64.

This patch is my proposed fix to PR rtl-optimization/123585, an ICE caused
by some incorrect logic in a gcc_assert.

2026-01-20  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR rtl-optimization/123585
	* simplify-rtx.cc (simplify_context::simplify_binary_operation_1)
	<case VEC_SELECT>: Correct gcc_assert when optimizing a vec_select
	of a vec_select with differing vector lengths.

gcc/testsuite/ChangeLog
	PR rtl-optimization/123585
	* gcc.target/i386/pr123585.c: New test case.