Move memory class instructions to new pass

* Refactored the Memory class of instructions in the spec out Id
validation and into a new pass
 * Tests unmodified
 * some minor disassembly changes
 * minor style changes
diff --git a/BUILD.gn b/BUILD.gn
index c9733b8..d79180a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -382,6 +382,7 @@
     "source/val/validate_layout.cpp",
     "source/val/validate_literals.cpp",
     "source/val/validate_logicals.cpp",
+    "source/val/validate_memory.cpp",
     "source/val/validate_non_uniform.cpp",
     "source/val/validate_primitives.cpp",
     "source/val/validate_type_unique.cpp",