make reciprocal estimate code generation more flexible by adding command-line options (2nd try)

The first try (r238051) to land this was reverted due to bot failures
that were hopefully addressed by r238788.

This patch adds a TargetRecip class for processing many recip codegen possibilities.
The class is intended to handle both command-line options to llc as well
as options passed in from a front-end such as clang with the -mrecip option.

The x86 backend is updated to use the new functionality.
Only -mcpu=btver2 with -ffast-math should see a functional change from this patch.
All other x86 CPUs continue to *not* use reciprocal estimates by default with -ffast-math.

Differential Revision: http://reviews.llvm.org/D8982

llvm-svn: 238842