rs6000: Avoid undefined behavior caused by overflow and invalid shifts

rs6000: Avoid undefined behavior caused by overflow and invalid shifts

While building GCC with --with-build-config=bootstrap-ubsan on
powerpc64le-unknown-linux-gnu, multiple UBSAN runtime errors were
encountered in rs6000.cc and rs6000.md due to undefined behavior
involving left shifts on negative values and shift exponents equal to
or exceeding the type width.

The issue was in bit pattern recognition code
(in can_be_rotated_to_negative_lis and can_be_built_by_li_and_rldic),
where signed values were shifted without handling negative inputs or
guarding against shift counts equal to the type width, causing UB.
The fix ensures shifts and rotations are done unsigned HOST_WIDE_INT,
and casting back only where needed (like for arithmetic right shifts)
with proper guards to prevent shift-by-64.

2025-07-31  Kishan Parmar  <kishan@linux.ibm.com>

gcc:
	PR target/118890
	* config/rs6000/rs6000.cc (can_be_rotated_to_negative_lis): Avoid left
	shift of negative value and guard shift count.
	(can_be_built_by_li_and_rldic): Likewise.
	(rs6000_emit_set_long_const): Likewise.
	* config/rs6000/rs6000.md (splitter for plus into two 16-bit parts): Fix
	UB from overflow in addition.