tree-ssa-ccp: Fix up bit_value_binop on RSHIFT_EXPR [PR102134]

As mentioned in the PR, this hunk is guarded with !wi::neg_p (r1val | r1mask, sgn)
which means if sgn is UNSIGNED, it is always true, but r1val | r1mask in
widest_int is still sign-extended.  That means wi::clz (arg) returns 0,
wi::get_precision (arg) returns some very large number
(WIDE_INT_MAX_PRECISION, on x86_64 576 bits) and width is 64, so we end up
with lzcount of -512 where the code afterwards expects a non-negative
lzcount.  For arg without the sign bit set the code works right, those
numbers are zero extended and so wi::clz must return wi::get_precision (arg) - width
plus number of leading zero bits within the width precision.
The patch fixes it by handling the sign-extension specially, either it could
be done through wi::neg_p (arg) check, but lzcount == 0 works identically.

2021-08-31  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/102134
	* tree-ssa-ccp.c (bit_value_binop) <case RSHIFT_EXPR>: If sgn is
	UNSIGNED and r1val | r1mask has MSB set, ensure lzcount doesn't
	become negative.

	* gcc.c-torture/execute/pr102134.c: New test.