AMDGPU/SIInsertWait: Skip dummy tied source

For D16 memory load instructions, the hardware usually only write to half
of the 32bit register, but we define the destination register using
32bit register for the MachineIR instruction. Without the extra tied
source register, LLVM framework will think previous write to the other
half of the register being dead. This is because by using 32bit register
as the destination register, LLVM will think the instruction will always
overwrite the whole 32bit register. By adding the extra tied source,
LLVM will think we are reading the register, so previous write to the
register will not be dead. This dummy tied source is introducing
unnecessary read-after-write dependency. The change here is to bypass the
tied source that can be skipped, thus avoiding an unnecessary s_waitcnt.

Reviewed by: foad

Differential Revision: https://reviews.llvm.org/D140537