[AMDGPU][MC] Restore old error position for "too few operands"

Revert part of https://reviews.llvm.org/D92084 to make it simpler to
start consuming the EndOfStatement token within AMDGPU's
ParseInstruction in a future patch. This also brings us back to what
every other target currently does.

A future change to move the position back to the end of the statement
would likely need to audit all of the AMDGPUOperand SMLoc ranges, and
determine the SMLoc for the last character of the last operand.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D92960