PR tree-optimization/113673: Avoid load merging when potentially trapping.

PR tree-optimization/113673: Avoid load merging when potentially trapping.

This patch fixes PR tree-optimization/113673, a P2 ice-on-valid regression
caused by load merging of (ptr[0]<<8)+ptr[1] when -ftrapv has been
specified.  When the operator is | or ^ this is safe, but for addition
of signed integer types, a trap may be generated/required, so merging this
idiom into a single non-trapping instruction is inappropriate, confusing
the compiler by transforming a basic block with an exception edge into one
without.

This revision implements Richard Biener's feedback to add an early check
for stmt_can_throw_internal (cfun, stmt) to prevent transforming in the
presence of any statement that could trap, not just overflow on addition.
The one other tweak included in this patch is to mark the local function
find_bswap_or_nop_load as static ensuring that it isn't called from outside
this file, and guaranteeing that it is dominated by stmt_can_throw_internal
checking.

2024-06-24  Roger Sayle  <roger@nextmovesoftware.com>
	    Richard Biener  <rguenther@suse.de>

gcc/ChangeLog
	PR tree-optimization/113673
	* gimple-ssa-store-merging.cc (find_bswap_or_nop_load): Make static.
	(find_bswap_or_nop_1): Avoid transformations (load merging) when
	stmt_can_throw_internal indicates that a statement can trap.

gcc/testsuite/ChangeLog
	PR tree-optimization/113673
	* g++.dg/pr113673.C: New test case.