Reduce the number of emitted local variables in C

This commit does various things oriented at reducing the number of local
variables emitted in C:
- MarkAssignments now know that @Copy and @Assign involving
  @LocalVariable only have side effects that affect the local variable
  itself; this enables to reduce the number of times we're forced to
  emit a local variable due to interfering side effects
- Drop the @AssignmentMarker FunctionTag; AddAssignmentMarkerPass now
  doesn't emit @AssignmentMarker anymore; instead it emits groups of
  @LocalVariable, @Copy, and @Assign, which benefit from the previous
  point
- Drop 2 MarkAssignments::Reasons: HasManyUses and HasUsesOutsideOfBB;
  both these have now been aggregated into the AlwaysAssign reason for
  simplicity, representing all reasons non involving side effects
- Update BeautifyGHAST and how it reasons about side effects when
  beautifying; before this commit it used @AssignmentMarker, now it
  looks at @Assign
- Simplify ExitSSA; before this commit it was trying hard to be smart on
  where it emitted the store instructions representing the incoming
  values of the PHI that was being destroyed; this seemed smart when we
  originally did it but it generated C code that was not really better
  to read, so this useless complexity is finally gone

lib/ValueManipulationAnalysis/TypeFlowGraph.cpp

@@ -413,7 +413,6 @@ static bool connect(TypeFlowNode *N1, TypeFlowNode *N2) {
 
         // These opcodes are transparent
         if (FunctionTags::Parentheses.isTagOf(Callee)
-            or FunctionTags::AssignmentMarker.isTagOf(Callee)
             or FunctionTags::Copy.isTagOf(Callee))
           return AddBidirectionalEdge(UseNode, ValNode, ALL_COLORS);
       }