[PATCH] [mips][microMIPS] Implement ERET and ERETNC instructions

[Daniel Sanders]

================
Comment at: lib/Target/Mips/MicroMips32r6InstrFormats.td:186-204
@@ -185,1 +185,20 @@
 }
+
+class ERET_FM_MMR6<string instr_asm, bits<10> funct> : MMR6Arch<instr_asm> {
+  bits<32> Inst;
+
+  let Inst{31-26} = 0x00;
+  let Inst{25-16} = 0x00;
+  let Inst{15-6}  = funct;
+  let Inst{5-0}   = 0x3c;
+}
+
+class ERETNC_FM_MMR6<string instr_asm, bits<10> funct> : MMR6Arch<instr_asm> {
+  bits<32> Inst;
+
+  let Inst{31-26} = 0x00;
+  let Inst{25-17} = 0x00;
+  let Inst{16-16} = 0x01;
+  let Inst{15-6}  = funct;
+  let Inst{5-0}   = 0x3c;
+}
----------------
dsanders wrote:
> Nit: Each of these two formats only have one instruction using them. As such, we should remove the 'funct' argument and define the number here.
One more nit. 'instr_asm' is unused and should be removed.

http://reviews.llvm.org/D10091

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