Use FrameIndexTy for stack protector

Using getValueType() is not correct for architectures extended with CHERI since
we need a pointer type and not the value that is loaded. While stack
protector is useless when you have CHERI (since CHERI provides much
stronger security guarantees), we still have a test to check that we can
generate correct code for checks. Merging b281138a
into our tree broke this test. Fix by using TLI.getFrameIndexTy().

Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D77785