tree-switch-conversion: Bugfixes for _BitInt [PR113491]

tree-switch-conversion: Bugfixes for _BitInt [PR113491]

The following patch fixes various issues with large/huge _BitInt used as switch
expressions.
In particular:
1) the indexes in CONSTRUCTORs shouldn't be types with precision larger than
   sizetype precision, varasm uses wi::to_offset on those and too large
   indexes ICE; we've already checked earlier that the cluster is at most
   sizetype bits and arrays can't be larger than that anyway
2) some spots were using SCALAR_INT_TYPE_MODE or
   lang_hooks.types.type_for_mode on TYPE_MODE to determine types to use,
   that obviously doesn't work for the large/huge BITINT_TYPE
3) like the recent change in the C FE, this patch makes sure we don't create
   ARRAY_REFs with indexes with precision above sizetype precision, because
   bitint lowering isn't prepared for that and because the indexes can't be
   larger than sizetype anyway; the subtraction of the cluster minimum from
   the index obviously needs to be done in unsigned __int128 or large/huge
   BITINT_TYPE, but then we cast to sizetype if the precision is larger than
   sizetype

2024-01-20  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/113491
	* tree-switch-conversion.cc (switch_conversion::build_constructors):
	If elt.index has precision higher than sizetype, fold_convert it to
	sizetype.
	(switch_conversion::array_value_type): Return type if type is
	BITINT_TYPE with precision above MAX_FIXED_MODE_SIZE or with BLKmode.
	(switch_conversion::build_arrays): Use unsigned_type_for rather than
	lang_hooks.types.type_for_mode if utype is BITINT_TYPE with precision
	above MAX_FIXED_MODE_SIZE or with BLKmode.  If utype has precision
	higher than sizetype, use sizetype as tidx type and fold_convert the
	subtraction to sizetype.

	* gcc.dg/torture/bitint-51.c: New test.