Improve code for uniform switches with a 'break' under varying control flow.

Previously, when we had a switch statement with a uniform switch condition
but a 'break' statement that was under varying control flow inside the
switch, we'd promote the switch condition to be varying so that the
break would work correctly.

Now, we leave the condition as uniform and are thus able to use the
more-efficient LLVM switch instruction in this case.

Issue #156.

stmt.cpp

@@ -2335,7 +2335,9 @@ SwitchStmt::EmitCode(FunctionEmitContext *ctx) const {
         return;
     }
 
-    ctx->StartSwitch(type->IsUniformType(), bbDone);
+    bool isUniformCF = (type->IsUniformType() &&
+                        lHasVaryingBreakOrContinue(stmts) == false);
+    ctx->StartSwitch(isUniformCF, bbDone);
     ctx->SwitchInst(exprValue, svi.defaultBlock ? svi.defaultBlock : bbDone,
                     svi.caseBlocks, svi.nextBlock);
 
@@ -2375,15 +2377,7 @@ SwitchStmt::TypeCheck() {
                     exprType->GetAsUniformType() == 
                     AtomicType::UniformConstInt64);
 
-    // FIXME: if there's a break or continue under varying control flow
-    // within a switch with a "uniform" condition, we promote the condition
-    // to varying so that everything works out and we are set to handle the
-    // resulting divergent control flow.  This is somewhat sub-optimal; see
-    // https://github.com/ispc/ispc/issues/156 for details.
-    bool isUniform = (exprType->IsUniformType() && 
-                      lHasVaryingBreakOrContinue(stmts) == false);
-
-    if (isUniform) {
+    if (exprType->IsUniformType()) {
         if (is64bit) toType = AtomicType::UniformInt64;
         else         toType = AtomicType::UniformInt32;
     }