[PATCH] [mips][mips64r6] Use JALR for returns instead of JR (which is not available on MIPS32r6/MIPS64r6)

[Daniel Sanders]

Hi jkolek, vmedic, zoran.jovanovic,

RET, and RET_MM have been replaced by a pseudo named PseudoReturn.
In addition a version with a 64-bit GPR named PseudoReturn64 has been
added.

Instruction selection for a return matches RetRA, which is expanded post
register allocation to PseudoReturn/PseudoReturn64. During MipsAsmPrinter,
this PseudoReturn/PseudoReturn64 are emitted as:
- (JALR64 $zero, $rs) on MIPS64r6
- (JALR $zero, $rs) on MIPS32r6
- (JR_MM $rs) on microMIPS
- (JR $rs) otherwise

On MIPS32r6/MIPS64r6, 'jr $rs' is an alias for 'jalr $zero, $rs'. To aid
development and review (specifically, to ensure all cases of jr are
updated), these aliases are temporarily named 'r6.jr' instead of 'jr'.
A follow up patch will change them back to the correct mnemonic.

Added (JALR $zero, $rs) to MipsNaClELFStreamer's definition of an indirect
jump, and removed it from its definition of a call.
Note: I haven't accounted for MIPS64 in MipsNaClELFStreamer since it's
doesn't appear to account for any MIPS64-specifics.

The return instruction created as part of eh_return expansion is now expanded
using expandRetRA() so we use the right return instruction on MIPS32r6/MIPS64r6
('jalr $zero, $rs').

Also, fixed a misuse of isABI_N64() to detect 64-bit wide registers in
expandEhReturn().

Depends on D4267

http://reviews.llvm.org/D4268

Files:
  lib/Target/Mips/MCTargetDesc/MipsNaClELFStreamer.cpp
  lib/Target/Mips/MicroMipsInstrInfo.td
  lib/Target/Mips/Mips32r6InstrInfo.td
  lib/Target/Mips/Mips64InstrInfo.td
  lib/Target/Mips/Mips64r6InstrInfo.td
  lib/Target/Mips/MipsAsmPrinter.cpp
  lib/Target/Mips/MipsAsmPrinter.h
  lib/Target/Mips/MipsInstrInfo.td
  lib/Target/Mips/MipsSEInstrInfo.cpp
  lib/Target/Mips/MipsSEInstrInfo.h
  test/CodeGen/Mips/eh-return32.ll
  test/CodeGen/Mips/eh-return64.ll
  test/CodeGen/Mips/llvm-ir/ret.ll
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