[AArch64] Add an option to reserve physical registers from RA

This patch adds an option --reserve-regs-for-regalloc, so we can reserve a list
of physical registers. These registers will not be used by register allocator,
but can still be used as ABI requests such as passing arguments to function
call.

Its main purpose is simulating high register pressure by reserving many physical
registers. So it will be much easier to test and debug register allocation
changes.

Differential Revision: https://reviews.llvm.org/D132717

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