[wasm] Remove obsolete DCHECKS

WasmGraphBuilder::GrowMemory does not access the module or the instance
any more. This was initially needed to reference the context as a
HeapConstant in the code.
This CL just removes the DCHECKs, which failed with the
--wasm-lazy-compilation flag.

R=ahaas@chromium.org
BUG=chromium:726665

Change-Id: Ieac53fe376256c47e8ef2fafca818a99ff063683
Reviewed-on: https://chromium-review.googlesource.com/516706Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45556}

src/compiler/wasm-compiler.cc

@@ -1705,11 +1705,6 @@ Node* WasmGraphBuilder::BuildFloatToIntConversionInstruction(
 }
 
 Node* WasmGraphBuilder::GrowMemory(Node* input) {
-  // GrowMemory will not be called from asm.js, hence we cannot be in
-  // lazy-compilation mode, hence the instance will be set.
-  DCHECK_NOT_NULL(module_);
-  DCHECK_NOT_NULL(module_->instance);
-
   Diamond check_input_range(
       graph(), jsgraph()->common(),
       graph()->NewNode(jsgraph()->machine()->Uint32LessThanOrEqual(), input,