Do not crash when tryingto fold unsupported spec constant (KhronosGroup#5496)

Remove assertion in FoldWithInstructionFolder; there are cases where
folding spec constants is unsupported.

Closes KhronosGroup#5492.

Author: ncesario-lunarg

source/opt/fold_spec_constant_op_and_composite_pass.cpp

@@ -176,8 +176,9 @@ Instruction* FoldSpecConstantOpAndCompositePass::FoldWithInstructionFolder(
   Instruction* new_const_inst =
       context()->get_instruction_folder().FoldInstructionToConstant(
           inst.get(), identity_map);
-  assert(new_const_inst != nullptr &&
-         "Failed to fold instruction that must be folded.");
+
+  // new_const_inst == null indicates we cannot fold this spec constant
+  if (!new_const_inst) return nullptr;
 
   // Get the instruction before |pos| to insert after.  |pos| cannot be the
   // first instruction in the list because its type has to come first.

test/opt/fold_spec_const_op_composite_test.cpp

@@ -674,6 +674,31 @@ TEST_F(FoldSpecConstantOpAndCompositePassBasicTest, CompositeInsertMatrixNull) {
   SinglePassRunAndMatch<FoldSpecConstantOpAndCompositePass>(test, false);
 }
 
+// Silently ignore spec constants that cannot be folded
+TEST_F(FoldSpecConstantOpAndCompositePassBasicTest, UnfoldableOp) {
+  const std::string test = R"(
+               OpCapability Shader
+               OpCapability SignedZeroInfNanPreserve
+               OpExtension "SPV_KHR_float_controls"
+               OpMemoryModel Logical GLSL450
+               OpEntryPoint Vertex %main "main"
+               OpSource GLSL 450
+               OpDecorate %v SpecId 1
+       %void = OpTypeVoid
+          %3 = OpTypeFunction %void
+       %float = OpTypeFloat 32
+          %v = OpConstant %float 0x1p-1
+%c = OpSpecConstantOp %float QuantizeToF16 %v
+;CHECK: {{%\w+}} = OpSpecConstantOp {{%\w+}} QuantizeToF16 {{%\w+}}
+       %main = OpFunction %void None %3
+          %5 = OpLabel
+               OpReturn
+               OpFunctionEnd
+)";
+
+  SinglePassRunAndMatch<FoldSpecConstantOpAndCompositePass>(test, false);
+}
+
 // All types and some common constants that are potentially required in
 // FoldSpecConstantOpAndCompositeTest.
 std::vector<std::string> CommonTypesAndConstants() {