Fix a logic bug when copying fast-math flags.

"Setting" does not equal "copying". This bug has sat dormant for 2 reasons:
1. The unit test was not adequate.
2. Every current user of the "copyFastMathFlags" API is operating on a new instruction.
   (ie, all existing fast-math flags are off). If you copy flags to an existing
   instruction that has some flags on already, you will not necessarily turn them off
   as expected.

I uncovered this bug while trying to implement a fix for PR20802.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216939 91177308-0d34-0410-b5e6-96231b3b80d8