Fix tree-ssa/pr31261.c testcase after r16-400 [PR120168]

Fix tree-ssa/pr31261.c testcase after r16-400 [PR120168]

AFter r16-400-g5e363ffefaceb9, on targets where char is unsigned by
default, tree-ssa/pr31261.c testcase started to fail:
FAIL: gcc.dg/tree-ssa/pr31261.c scan-tree-dump-times original "return \\\\(char\\\\) -\\\\(unsigned char\\\\) c & 31;" 1

This is because the casts are no longer needed as both char and
unsigned char are the same signedness.
I was deciding between add -fsigned-char or changing the testcase
to use explicitly `signed char`. I went with using an explicit
`signed char` as that would be case normally.

	PR testsuite/120168

gcc/testsuite/ChangeLog:

	* gcc.dg/tree-ssa/pr31261.c: Use `signed char` instead
	of plain char.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>