cse: Workaround GCC < 5 bug in cse_insn [PR111852]

cse: Workaround GCC < 5 bug in cse_insn [PR111852]

Before the r5-3834 commit for PR63362, GCC 4.8-4.9 refuses to compile
cse.cc which contains a variable with rtx_def type, because rtx_def
contains a union with poly_uint16 element.  poly_int template has
defaulted default constructor and a variadic template constructor which
could have empty parameter pack. GCC < 5 treated it as non-trivially
constructible class and deleted rtunion and rtx_def default constructors.

For the cse_insn purposes, all we need is a variable with size and alignment
of rtx_def, not necessarily rtx_def itself, which we then memset to 0 and
fill in like rtx is normally allocated from heap, so this patch for
GCC_VERSION < 5000 uses an unsigned char array of the right size/alignment.

2023-10-18  Jakub Jelinek  <jakub@redhat.com>

	PR bootstrap/111852
	* cse.cc (cse_insn): Add workaround for GCC 4.8-4.9, instead of
	using rtx_def type for memory_extend_buf, use unsigned char
	arrayy with size of rtx_def and its alignment.

(cherry picked from commit bc4bd69faf986326f6b0fd0400cdd6871577afd1)