[NFC][X86][AArch64] Negative tests for 'check for [no] signed truncation' pattern

See D49247, D49266

I'm only adding the sane negative tests, and not
adding the one-use tests yet. Also, not adding
negative tests for the second pattern with inverted operands yet,
since it's handling will be added in later differential.

llvm-svn: 337014