[MC] evaluateAsAbsolute requires MCValue::RefKind==0

In `.equ a, 3; .if a@plt`, a@plt does not evaluate to an absolute value
(MCExpr::evaluateAsRelocatableImpl disables evaluation when the Kind !=
0 at parse time). Similarly, when using MCTargetValue,
evaluateAsAbsolute should return false when MCValue::RefKind==0.

This allows us to remove `if (!Asm)` check from MipsMCExpr.cpp
(%hi(0xdeadbeef) is not evaluated to a constant without RefKind) and
make targets less error-prone.

Author: MaskRay

llvm/lib/MC/MCExpr.cpp

@@ -286,11 +286,11 @@ bool MCExpr::evaluateAsAbsolute(int64_t &Res, const MCAssembler *Asm,
   }
 
   bool IsRelocatable = evaluateAsRelocatableImpl(Value, Asm, Addrs, InSet);
-
-  // Record the current value.
   Res = Value.getConstant();
-
-  return IsRelocatable && Value.isAbsolute();
+  // Value with RefKind (e.g. %hi(0xdeadbeef) in MIPS) is not considered
+  // absolute (the value is unknown at parse time), even if it might be resolved
+  // by evaluateFixup.
+  return IsRelocatable && Value.isAbsolute() && Value.getRefKind() == 0;
 }
 
 /// Helper method for \see EvaluateSymbolAdd().

llvm/lib/Target/Mips/MCTargetDesc/MipsMCExpr.cpp

@@ -131,8 +131,6 @@ void MipsMCExpr::printImpl(raw_ostream &OS, const MCAsmInfo *MAI) const {
 
 bool MipsMCExpr::evaluateAsRelocatableImpl(MCValue &Res, const MCAssembler *Asm,
                                            const MCFixup *Fixup) const {
-  if (!Asm)
-    return false;
   // Look for the %hi(%neg(%gp_rel(X))) and %lo(%neg(%gp_rel(X))) special cases.
   if (isGpOff()) {
     const MCExpr *SubExpr =