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

tests/verilog/mem_bounds.sv

@@ -0,0 +1,27 @@
+module top;
+    reg [0:7] mem [0:2];
+
+    initial mem[1] = '1;
+    wire [31:0] a, b, c, d;
+    assign a = mem[1];
+    assign b = mem[-1];
+    assign c = mem[-1][0];
+    assign d = mem[-1][0:1];
+
+    always @* begin
+
+    	assert ($countbits(a, '0) == 24);
+    	assert ($countbits(a, '1) == 8);
+    	assert ($countbits(a, 'x) == 0);
+
+    	assert ($countbits(b, '0) == 24);
+    	assert ($countbits(b, 'x) == 8);
+
+    	assert ($countbits(c, '0) == 31);
+    	assert ($countbits(c, 'x) == 1);
+
+    	assert ($countbits(d, '0) == 30);
+    	assert ($countbits(d, 'x) == 2);
+
+    end
+endmodule

tests/verilog/mem_bounds.ys

@@ -0,0 +1,6 @@
+read_verilog -sv -mem2reg mem_bounds.sv
+proc
+flatten
+opt -full
+select -module top
+sat -verify -seq 1 -tempinduct -prove-asserts -show-all -enable_undef