recognize some patterns as fabs operations, so that fabs at the source level

recognize some patterns as fabs operations, so that fabs at the source level
is deconstructed then reconstructed here.  This catches 19 fabs's in 177.mesa
9 in 168.wupwise, 5 in 171.swim, 3 in 172.mgrid, and 14 in 173.applu out of
specfp2000.

This allows the X86 code generator to make MUCH better code than before for
each of these and saves one instr on ppc.

This depends on the previous CFE patch to expose these correctly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@21171 91177308-0d34-0410-b5e6-96231b3b80d8