make mcinstlower remove all but the first operand to CALL64pcrel32.

The register use operands (e.g. the first argument is passed in a 
register) is currently being modeled as a normal register use,
instead of correctly being an implicit use.  This causes the operand
to get propagated onto the mcinst, which was causing the encoder to
emit a rex prefix byte, which generates an invalid call.

This fixes rdar://7998435

llvm-svn: 104062