Allow x87 FP registers to be alive globally in a function.

FP_REG_KILL instructions are still inserted, but can be disabled by passing
-live-x87 to llc. The X87FPRegKillInserterPass is going to be removed shortly.

CFG edges are partioned into bundles where the x87 stack must be allocated
identically. Code is insertad at the end of each basic block that shuffles the
live FP registers to match the outgoing bundles expectations.

This fix is in preparation for some upcoming register allocator improvements
that may extend the live range of registers beyond a basic block, similar to
LICM. It also provides a nice runtime speedup if you are building with
-mfpmath=387.

llvm-svn: 108529