[NFC] Fix check prefix add in fcanonicalize-elimination.ll

The test fix added by "D39306: Fix
CodeGen/AMDGPU/fcanonicalize-elimination.ll on FreeBSD 11.0" uses a test
prefix which is not actually used in the FileCheck stanza. Thus the
problem originally encountered still exists and the tests fails for host
triples that  contain "1.0", including AIX 7.1.0.