[AMDGPU] Allow multiple uses of the same literal

In GFX10 VOP3 can have a literal, which opens up the possibility of two
operands using the same literal value, which is allowed and only counts
as one use of the constant bus.

AMDGPUAsmParser::validateConstantBusLimitations already knew about this
but SIInstrInfo::verifyInstruction did not.

Differential Revision: https://reviews.llvm.org/D100770