Fix accessing memory reference types in yul codegen and clean up on memory reads.

test/libsolidity/semanticTests/viaYul/dirty_memory_read.sol

@@ -0,0 +1,15 @@
+contract C {
+	function f() public pure returns (uint8 x, bool a, bool b) {
+		uint8[1] memory m;
+		assembly {
+			mstore(m, 257)
+		}
+		x = m[0];
+		a = (m[0] == 0x01);
+		b = (m[0] == 0x0101);
+	}
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 1, true, false