Change cast to dyn_cast to be consistent with other casts within same scope.

[BrandonTJones]

Summary

This cast seems incorrect looking at similar and may cause errors.

[dblaikie]

llvm/lib/MC/MCExpr.cpp

764

If this dyn_cast is never false (if the original code never hit an assert) then the 'if' condition should be removed, and the cast should remain as a cast (not a dyn_cast)

If there are cases where the RHS is not an MCTargetExpr (even though the LHS is) - then this change should include a test case that demonstrates/exercises that case.

llvm/lib/MC/MCExpr.cpp

case Binary: {
     MCValue LHSValue, RHSValue;
 
     if (!ABE->getLHS()->evaluateAsRelocatableImpl(LHSValue, Asm, Layout, Fixup,
                                                   Addrs, InSet) ||
         !ABE->getRHS()->evaluateAsRelocatableImpl(RHSValue, Asm, Layout, Fixup,
                                                   Addrs, InSet)) {
       // Check if both are Target Expressions, see if we can compare them.
       if (const MCTargetExpr *L = dyn_cast<MCTargetExpr>(ABE->getLHS()))
-        if (const MCTargetExpr *R = cast<MCTargetExpr>(ABE->getRHS())) {
+        if (const MCTargetExpr *R = dyn_cast<MCTargetExpr>(ABE->getRHS())) {
           switch (ABE->getOpcode()) {
           case MCBinaryExpr::EQ:
             Res = MCValue::get((L->isEqualTo(R)) ? -1 : 0);
             return true;
           case MCBinaryExpr::NE:
             Res = MCValue::get((R->isEqualTo(R)) ? 0 : -1);
             return true;
           default: break;