Fix ARMFastISel::ARMEmitIntExt shift emission

My patch 'r183551 - ARM FastISel integer sext/zext improvements' was incorrect when emitting ARM register-immediate ASR, LSL, LSR instructions: they are pseudo-instructions in ARMInstrInfo.td and I should have used MOVsi instead.

This is not an issue when code is generated through a .s file, but is an issue when generated straight to a .o (-filetype=obj).