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

[hfinkel at anl.gov]

LGTM (pending the LLVM change obviously).


================
Comment at: lib/Driver/Tools.cpp:1635
@@ +1634,3 @@
+    char RefStepChar = RefStep[0];
+    if (RefStepChar < '0' || RefStepChar > '9') {
+      D.Diag(diag::err_drv_invalid_value) << Option << RefStep;
----------------
Please add a comment here explaining that this is reasonable because for currently-supported operations on currently-supported architectures, reasonable value here are 0 through 5 or 6 (and 9 is certainly overkill). We can extend this to support larger numbers as necessary in the future.

http://reviews.llvm.org/D8989

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