change an assert when generating fmuladd to an ordinary 'if' check (PR25719)

change an assert when generating fmuladd to an ordinary 'if' check (PR25719)

We don't want to generate fmuladd if there's a use of the fmul expression, but this shouldn't be an assert.

The test case is derived from the commit message for r253337:
http://reviews.llvm.org/rL253337

That commit reverted r253269:
http://reviews.llvm.org/rL253269

...but the bug exists independently of the default fp-contract setting. It just became easier to hit with that change.

PR25719:
https://llvm.org/bugs/show_bug.cgi?id=25719

Differential Revision: http://reviews.llvm.org/D15165

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@254573 91177308-0d34-0410-b5e6-96231b3b80d8