bitint: Avoid rewriting large/huge _BitInt vars into SSA after bitint lowering [PR114278]

bitint: Avoid rewriting large/huge _BitInt vars into SSA after bitint lowering [PR114278]

The following testcase ICEs, because update-address-taken subpass of
fre5 rewrites
  _BitInt(128) b;
  vector(16) unsigned char _3;

  <bb 2> [local count: 1073741824]:
  _3 = MEM <vector(16) unsigned char> [(char * {ref-all})p_2(D)];
  MEM <vector(16) unsigned char> [(char * {ref-all})&b] = _3;
  b ={v} {CLOBBER(eos)};
to
  _BitInt(128) b;
  vector(16) unsigned char _3;

  <bb 2> [local count: 1073741824]:
  _3 = MEM <vector(16) unsigned char> [(char * {ref-all})p_2(D)];
  b_5 = VIEW_CONVERT_EXPR<_BitInt(128)>(_3);
but we can't have large/huge _BitInt vars in SSA form after the bitint
lowering except for function arguments loaded from memory, as expansion
isn't able to deal with those, it relies on bitint lowering to lower
those operations.
The following patch fixes that by setting DECL_NOT_GIMPLE_REG_P for
large/huge _BitInt vars after bitint lowering, such that we don't
rewrite them into SSA form.

2024-03-11  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/114278
	* tree-ssa.cc (maybe_optimize_var): If large/huge _BitInt vars are no
	longer addressable, set DECL_NOT_GIMPLE_REG_P on them.

	* gcc.dg/bitint-99.c: New test.