Tweak new test cases for -march=cascadelake strangeness.

Tweak new test cases for -march=cascadelake strangeness.

As reported by Sunil's tester, -march=cascadelake triggers some SUBREG
non-determinacy in the generated assembler for my new tests.  Fixed
by updating the regular expressions to match either the zero or sign
extended forms.  I'm testing a backend patch that may help with the
underlying cause of these differences.

2021-10-08  Roger Sayle  <roger@nextmovesoftware.com>

gcc/testsuite/ChangeLog
	* gcc.target/i386/sse2-mmx-paddsb-2.c: Test for -128 or 128.
	* gcc.target/i386/sse2-mmx-paddusb-2.c: Test for -1 or 255.
	* gcc.target/i386/sse2-mmx-psubsb-2.c: Test for -128 or 128.