[spirv-reader] Start emitting expression

Bug: tint:3
Change-Id: I2e2fe41c43abbcbbc941968c0115d703f6bff501
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/18822
Reviewed-by: dan sinclair <dsinclair@google.com>

src/reader/spirv/function.cc

diff --git a/src/reader/spirv/function.cc b/src/reader/spirv/function.cc
index 22dc974..6cb34f0 100644
--- a/src/reader/spirv/function.cc
+++ b/src/reader/spirv/function.cc
@@ -163,6 +163,18 @@
   return success();
 }
 
+std::unique_ptr<ast::Expression> FunctionEmitter::MakeExpression(uint32_t id) {
+  if (failed()) {
+    return nullptr;
+  }
+  const auto* spirv_constant = constant_mgr_->FindDeclaredConstant(id);
+  if (spirv_constant) {
+    return parser_impl_.MakeConstantExpression(id);
+  }
+  Fail() << "unhandled expression for ID " << id;
+  return nullptr;
+}
+
 }  // namespace spirv
 }  // namespace reader
 }  // namespace tint

src/reader/spirv/function.h

diff --git a/src/reader/spirv/function.h b/src/reader/spirv/function.h
index 7a4dd95..6a00f30 100644
--- a/src/reader/spirv/function.h
+++ b/src/reader/spirv/function.h
@@ -66,6 +66,11 @@
   /// @returns false if emission failed.
   bool EmitFunctionVariables();
 
+  /// Makes an expression
+  /// @param id the SPIR-V ID of the value
+  /// @returns true if emission has not yet failed.
+  std::unique_ptr<ast::Expression> MakeExpression(uint32_t id);
+
  private:
   /// @returns the store type for the OpVariable instruction, or
   /// null on failure.