[AMDGPU] Improve the lowering of raw_buffer_load_{i8,i16} and...

[AMDGPU] Improve the lowering of raw_buffer_load_{i8,i16} and struct_buffer_load_{i8,i16} intrinsics

Currently, raw_buffer_load_{i8,i16} and struct_buffer_load_{i8,i16}
intrinsics are lowered as buffer_load_{u8,u16}. This patch combines
buffer_load_{u8,u16} and sign extension instructions in order to
generate buffer_load_{i8,i16} instructions.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D144313