Revert "Make FP_CONTRACT ON the default."

Revert "Make FP_CONTRACT ON the default."

This reverts commit r253269.

This leads to assert / segfault triggering on the following reduced example:
float foo(float U, float base, float cell) { return (U = 2 * base) - cell; }

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@253337 91177308-0d34-0410-b5e6-96231b3b80d8