Rename memrr ptrreg and offreg components.

Currently, the sub-operand of a memrr address that corresponds to what
hardware considers the base register is called "offreg", while the
sub-operand that corresponds to the offset is called "ptrreg".

To avoid confusion, this patch simply swaps the named of those two
sub-operands and updates all uses.  No functional change is intended.

llvm-svn: 177734