Make AArch64FastISel::EmitIntExt explicitly check its source and destination types

This is a follow up to r212492. There should be no functional difference, but
this patch makes it clear that SrcVT must be an i1/i8/16/i32 and DestVT must be
an i8/i16/i32/i64.

rdar://17516686

llvm-svn: 212633