[X86] Remove mergeSPUpdatesUp()

X86FrameLowering has both a mergeSPUpdates() that accepts a direction, and an
mergeSPUpdatesUp(), which seem to do the same thing, except for a slightly 
different interface. Removed the less general function.
NFC.

Differential Revision: http://reviews.llvm.org/D11510

llvm-svn: 243396