[PowerPC] Mark many instructions as commutative

I'm under the impression that we used to infer the isCommutable flag from the
instruction-associated pattern. Regardless, we don't seem to do this (at least
by default) any more. I've gone through all of our instruction definitions, and
marked as commutative all of those that should be trivial to commute (by
exchanging the first two operands). There has been special code for the RL*
instructions, and that's not changed.

Before this change, we had the following commutative instructions:

 RLDIMI
 RLDIMIo
 RLWIMI
 RLWIMI8
 RLWIMI8o
 RLWIMIo
 XSADDDP
 XSMULDP
 XVADDDP
 XVADDSP
 XVMULDP
 XVMULSP

After:

 ADD4
 ADD4o
 ADD8
 ADD8o
 ADDC
 ADDC8
 ADDC8o
 ADDCo
 ADDE
 ADDE8
 ADDE8o
 ADDEo
 AND
 AND8
 AND8o
 ANDo
 CRAND
 CREQV
 CRNAND
 CRNOR
 CROR
 CRXOR
 EQV
 EQV8
 EQV8o
 EQVo
 FADD
 FADDS
 FADDSo
 FADDo
 FMADD
 FMADDS
 FMADDSo
 FMADDo
 FMSUB
 FMSUBS
 FMSUBSo
 FMSUBo
 FMUL
 FMULS
 FMULSo
 FMULo
 FNMADD
 FNMADDS
 FNMADDSo
 FNMADDo
 FNMSUB
 FNMSUBS
 FNMSUBSo
 FNMSUBo
 MULHD
 MULHDU
 MULHDUo
 MULHDo
 MULHW
 MULHWU
 MULHWUo
 MULHWo
 MULLD
 MULLDo
 MULLW
 MULLWo
 NAND
 NAND8
 NAND8o
 NANDo
 NOR
 NOR8
 NOR8o
 NORo
 OR
 OR8
 OR8o
 ORo
 RLDIMI
 RLDIMIo
 RLWIMI
 RLWIMI8
 RLWIMI8o
 RLWIMIo
 VADDCUW
 VADDFP
 VADDSBS
 VADDSHS
 VADDSWS
 VADDUBM
 VADDUBS
 VADDUHM
 VADDUHS
 VADDUWM
 VADDUWS
 VAND
 VAVGSB
 VAVGSH
 VAVGSW
 VAVGUB
 VAVGUH
 VAVGUW
 VMADDFP
 VMAXFP
 VMAXSB
 VMAXSH
 VMAXSW
 VMAXUB
 VMAXUH
 VMAXUW
 VMHADDSHS
 VMHRADDSHS
 VMINFP
 VMINSB
 VMINSH
 VMINSW
 VMINUB
 VMINUH
 VMINUW
 VMLADDUHM
 VMULESB
 VMULESH
 VMULEUB
 VMULEUH
 VMULOSB
 VMULOSH
 VMULOUB
 VMULOUH
 VNMSUBFP
 VOR
 VXOR
 XOR
 XOR8
 XOR8o
 XORo
 XSADDDP
 XSMADDADP
 XSMAXDP
 XSMINDP
 XSMSUBADP
 XSMULDP
 XSNMADDADP
 XSNMSUBADP
 XVADDDP
 XVADDSP
 XVMADDADP
 XVMADDASP
 XVMAXDP
 XVMAXSP
 XVMINDP
 XVMINSP
 XVMSUBADP
 XVMSUBASP
 XVMULDP
 XVMULSP
 XVNMADDADP
 XVNMADDASP
 XVNMSUBADP
 XVNMSUBASP
 XXLAND
 XXLNOR
 XXLOR
 XXLXOR

This is a by-inspection change, and I'm not sure how to write a reliable test
case. I would like advice on this, however.