Currently, type casts can only be used to pattern match for intrinsics
with a single overloaded return value. For instance:
```
def int_foo : Intrinsic<[llvm_anyint_ty], []>;
def : Pat<(i32 (int_foo)), ...>;
```
This patch extends type casts to support matching intrinsics with
multiple overloaded return values. As an example, the following defines
a pattern that matches only if the overloaded intrinsic call returns an
`i16` for the first result and an `i32` for the second result:
```
def int_bar : Intrinsic<[llvm_anyint_ty, llvm_anyint_ty], []>;
def : Pat<([i16, i32] (int_bar)), ...>;
```
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