Fix fixup evaluation when deciding what to relocate with.

The previous logic was to first try without relocations at all
and failing that stop on the first defined symbol.

That was inefficient and incorrect in the case part of the
expression could be simplified and another part could not
(see included test).

We now stop the evaluation when we get to a variable whose value
can change (i.e. is weak).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@233187 91177308-0d34-0410-b5e6-96231b3b80d8

lib/MC/MCObjectWriter.cpp

@@ -54,3 +54,5 @@ MCObjectWriter::IsSymbolRefDifferenceFullyResolvedImpl(const MCAssembler &Asm,
   // On ELF and COFF  A - B is absolute if A and B are in the same section.
   return &SecA == &SecB;
 }
+
+bool MCObjectWriter::isWeak(const MCSymbolData &SD) const { return false; }