mem2reg: tolerate out of bounds constant accesses

This brings the mem2reg behavior in line with the nomem2reg behavior.

tests/simple/mem2reg_bounds_tern.v

@@ -0,0 +1,19 @@
+module top(
+    input clk,
+    input wire [1:0] sel,
+    input wire [7:0] base,
+    output reg [7:0] line
+);
+    reg [0:7] mem [0:2];
+
+    generate
+        genvar i;
+        for (i = 0; i < 4; i = i + 1) begin : gen
+            always @(posedge clk)
+                mem[i] <= i == 0 ? base : mem[i - 1] + 1;
+        end
+    endgenerate
+
+    always @(posedge clk)
+        line = mem[sel];
+endmodule