[x86] set default reciprocal (division and square root) codegen to match GCC

[x86] set default reciprocal (division and square root) codegen to match GCC

D8982 ( checked in at http://reviews.llvm.org/rL239001 ) added command-line
options to allow reciprocal estimate instructions to be used in place of
divisions and square roots.

This patch changes the default settings for x86 targets to allow that recip
codegen (except for scalar division because that breaks too much code) when
using -ffast-math or its equivalent.

This matches GCC behavior for this kind of codegen.

Differential Revision: http://reviews.llvm.org/D10396

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240310 91177308-0d34-0410-b5e6-96231b3b80d8