IR: Fix up doxygen comment for LLVMContext::diagnose

This comment was referring to the DiagnosticSeverity with RS_
prefixes, but they're actually DS_. I've also modernized the comment
style since I was changing it anyway.
[FastISel][AArch64] Fix shift lowering for i8 and i16 value types.

This fix changes the parameters #r and #s that are passed to the UBFM/SBFM
instruction to get the zero/sign-extension for free.

The original problem was that the shift left would use the 32-bit shift even for
i8/i16 value types, which could leave the upper bits set with "garbage" values.

The arithmetic shift right on the other side would use the wrong MSB as sign-bit
to determine what bits to shift into the value.

This fixes <rdar://problem/17907720>.