[AMDGPU] return Fail instead of SolfFail from addOperand()

addOperand() method of AMDGPU disassembler returns SoftFail
on error. All instances which may lead to that place are
an impossible encdoing, not something which is possible to
encode, but semantically incorrect as described for SoftFail.

Then tablegen generates a check of the following form:

if (Decode...(..) == MCDisassembler::Fail) { return MCDisassembler::Fail; }

Since we can only return Success and SoftFail that is dead
code as detected by the static code analyzer.

Solution: return Fail as it should be.

See https://bugs.llvm.org/show_bug.cgi?id=43886

Differential Revision: https://reviews.llvm.org/D69819