[X86] Add test for PR39926; NFC

The test file shows a case where the avoid store forwarding block
pass misses to copy a range (-1..1) when the load displacement
changes sign.

Baseline test for D55485.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348712 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed