Eliminate redundant CR moves on PPC32.

The 32-bit ABI requires CR bit 6 to be set if the call has fp arguments and
unset if it doesn't. The solution up to now was to insert a MachineNode to
set/unset the CR bit, which produces a CR vreg. This vreg was then copied
into CR bit 6. When the register allocator saw a bunch of these in the same
function, it allocated the set/unset CR bit in some random CR register (1
extra instruction) and then emitted CR moves before every vararg function
call, rather than just setting and unsetting CR bit 6 directly before every
vararg function call. This patch instead inserts a PPCcrset/PPCcrunset
instruction which are then matched by a dedicated instruction pattern.

Patch by Tobias von Koch.

llvm-svn: 162725