[LLDB][NFC] Remove redundant condition

Condition `auto_advance_pc` is checked both in an outer and in an
inner `if` statement in `EmulateInstructionARM::EvaluateInstruction()`,
`EmulateInstructionARM64::EvaluateInstruction()` and
`EmulateInstructionPPC64::EvaluateInstruction()`. This patch removes the
redundant inner check.

The issue was found using `clang-tidy` check under review
`misc-redundant-condition`. See https://reviews.llvm.org/D81272.

Differential Revision: https://reviews.llvm.org/D82558