Generate FSQRT from calls to the sqrt function, which

Generate FSQRT from calls to the sqrt function, which
allows appropriate backends to generate a sqrt instruction.

On x86, this isn't done at -O0 because we go through
FastISel instead.  This is a behavior change from before
this series of sqrt patches started.  I think this is OK
considering that compile speed is most important at -O0, but
could be convinced otherwise.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82778 91177308-0d34-0410-b5e6-96231b3b80d8