[PATCH] [x86] add the -mrecip driver flag and process its options

[Sanjay Patel]

Hi hfinkel, andreadb, alexr,

This is the front-end counterpart to D8982. 

The option parsing implementation is based on maintaining compatibility with gcc; gcc documentation for this flag:
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/i386-and-x86-64-Options.html#index-mrecip_003dopt-1627

Note that PowerPC could have its own -mrecip options if further compatibility is desired:
https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/RS_002f6000-and-PowerPC-Options.html#index-mrecip-2289

With this flag in place, customers will have fine-grain control over reciprocal estimate codegen, so we may choose to align with gcc's optimization behavior for that. Eg, LLVM could generate reciprocal square root by default when -ffast-math is on. This was requested in PR21385 ( https://llvm.org/bugs/show_bug.cgi?id=21385 ).

http://reviews.llvm.org/D8989

Files:
  include/clang/Driver/Options.td
  lib/Driver/Tools.cpp
  test/Driver/x86_features.c

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