Add pseudo instructions to the XCore for (load|store|load address) of a

frame index. eliminateFrameIndex will replace these instructions with
(LDWSP|STWSP|LDAWSP) or (LDW|STW|LDAWF) if a frame pointer is in use.

This fixes PR 3324. Previously we used LDWSP, STWSP, LDAWSP before frame
pointer elimination. However since they were marked as implicitly using
SP they could not be rematerialised.

llvm-svn: 62238