[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