[PowerPC] Support compare mnemonics with implied CR0

Just like for branch mnemonics (where support was recently added), the
assembler is supposed to support extended mnemonics for the compare
instructions where no condition register is specified explicitly
(and CR0 is assumed implicitly).

This patch adds support for those extended compare mnemonics.


Index: llvm-head/test/MC/PowerPC/ppc64-encoding-ext.s
===================================================================
--- llvm-head.orig/test/MC/PowerPC/ppc64-encoding-ext.s
+++ llvm-head/test/MC/PowerPC/ppc64-encoding-ext.s
@@ -449,21 +449,37 @@

 # CHECK: cmpdi 2, 3, 128                 # encoding: [0x2d,0x23,0x00,0x80]
          cmpdi 2, 3, 128
+# CHECK: cmpdi 0, 3, 128                 # encoding: [0x2c,0x23,0x00,0x80]
+         cmpdi 3, 128
 # CHECK: cmpd 2, 3, 4                    # encoding: [0x7d,0x23,0x20,0x00]
          cmpd 2, 3, 4
+# CHECK: cmpd 0, 3, 4                    # encoding: [0x7c,0x23,0x20,0x00]
+         cmpd 3, 4
 # CHECK: cmpldi 2, 3, 128                # encoding: [0x29,0x23,0x00,0x80]
          cmpldi 2, 3, 128
+# CHECK: cmpldi 0, 3, 128                # encoding: [0x28,0x23,0x00,0x80]
+         cmpldi 3, 128
 # CHECK: cmpld 2, 3, 4                   # encoding: [0x7d,0x23,0x20,0x40]
          cmpld 2, 3, 4
+# CHECK: cmpld 0, 3, 4                   # encoding: [0x7c,0x23,0x20,0x40]
+         cmpld 3, 4

 # CHECK: cmpwi 2, 3, 128                 # encoding: [0x2d,0x03,0x00,0x80]
          cmpwi 2, 3, 128
+# CHECK: cmpwi 0, 3, 128                 # encoding: [0x2c,0x03,0x00,0x80]
+         cmpwi 3, 128
 # CHECK: cmpw 2, 3, 4                    # encoding: [0x7d,0x03,0x20,0x00]
          cmpw 2, 3, 4
+# CHECK: cmpw 0, 3, 4                    # encoding: [0x7c,0x03,0x20,0x00]
+         cmpw 3, 4
 # CHECK: cmplwi 2, 3, 128                # encoding: [0x29,0x03,0x00,0x80]
          cmplwi 2, 3, 128
+# CHECK: cmplwi 0, 3, 128                # encoding: [0x28,0x03,0x00,0x80]
+         cmplwi 3, 128
 # CHECK: cmplw 2, 3, 4                   # encoding: [0x7d,0x03,0x20,0x40]
          cmplw 2, 3, 4
+# CHECK: cmplw 0, 3, 4                   # encoding: [0x7c,0x03,0x20,0x40]
+         cmplw 3, 4

 # FIXME: Trap mnemonics

Index: llvm-head/lib/Target/PowerPC/PPCInstrInfo.td
===================================================================
--- llvm-head.orig/lib/Target/PowerPC/PPCInstrInfo.td
+++ llvm-head/lib/Target/PowerPC/PPCInstrInfo.td
@@ -2201,3 +2201,12 @@ defm : BranchExtendedMnemonic<"ne", 68>;
 defm : BranchExtendedMnemonic<"nu", 100>;
 defm : BranchExtendedMnemonic<"ns", 100>;

+def : InstAlias<"cmpwi $rA, $imm", (CMPWI CR0, gprc:$rA, s16imm:$imm)>;
+def : InstAlias<"cmpw $rA, $rB", (CMPW CR0, gprc:$rA, gprc:$rB)>;
+def : InstAlias<"cmplwi $rA, $imm", (CMPLWI CR0, gprc:$rA, u16imm:$imm)>;
+def : InstAlias<"cmplw $rA, $rB", (CMPLW CR0, gprc:$rA, gprc:$rB)>;
+def : InstAlias<"cmpdi $rA, $imm", (CMPDI CR0, g8rc:$rA, s16imm:$imm)>;
+def : InstAlias<"cmpd $rA, $rB", (CMPD CR0, g8rc:$rA, g8rc:$rB)>;
+def : InstAlias<"cmpldi $rA, $imm", (CMPLDI CR0, g8rc:$rA, u16imm:$imm)>;
+def : InstAlias<"cmpld $rA, $rB", (CMPLD CR0, g8rc:$rA, g8rc:$rB)>;
+