[PATCH] [PPC64] Properly handle the mftb instruction

[Kit Barton]

Hi echristo, hfinkel, wschmidt, nemanjai, seurer,

The mftb instruction was incorrectly marked as deprecated in the PPC Backend. Instead, it should not be treated as deprecated, but rather be implemented using the mfspr instruction. A similar patch was put into GCC last year. Details can be found at: 
  https://sourceware.org/ml/binutils/2014-11/msg00383.html.

This change will replace instances of the mftb instruction with the mfspr instruction for all CPUs except 601 and pwr3. This will also be the default behaviour.

Additional details can be found in:
  https://llvm.org/bugs/show_bug.cgi?id=23680

http://reviews.llvm.org/D10419

Files:
  lib/Target/PowerPC/AsmParser/PPCAsmParser.cpp
  lib/Target/PowerPC/PPC.td
  lib/Target/PowerPC/PPCInstrInfo.td
  test/CodeGen/PowerPC/mftb.ll
  test/MC/PowerPC/deprecated-p7.s

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