[CodeGen] Add MVT::FIRST_VALUETYPE to avoid explicit 0. NFC.

Many places reference MVT::LAST_VALUETYPE when iterating over all
valid MVTs, but they usually start with 0.
With FIRST_VALUETYPE, we can avoid explicit constants when we really
should be using MVT::SimpleValueType.

llvm-svn: 225362

include/llvm/CodeGen/MachineValueType.h

@@ -118,6 +118,7 @@ namespace llvm {
                               // unspecified type.  The register class
                               // will be determined by the opcode.
 
+      FIRST_VALUETYPE = 0,    // This is always the beginning of the list.
       LAST_VALUETYPE =  58,   // This always remains at the end of the list.
 
       // This is the current maximum for LAST_VALUETYPE.