Intrinsics: expand semantics of LLVMExtendedVectorType (& trunc)

These are used in the ARM backends to aid type-checking on patterns involving
intrinsics. By making sure one argument is an extended/truncated version of
another.

However, there's no reason to limit them to just vectors types. For example
AArch64 has the instruction "uqshrn sD, dN, #imm" which would naturally use an
intrinsic taking an i64 and returning an i32.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205003 91177308-0d34-0410-b5e6-96231b3b80d8