Do not use shiftmul peepopt pattern when mul result is truncated, fixes #1047

Signed-off-by: Clifford Wolf <[email protected]>

tests/simple/peepopt.v

@@ -2,6 +2,10 @@ module peepopt_shiftmul_0 #(parameter N=3, parameter W=3) (input [N*W-1:0] i, in
 assign o = i[s*W+:W];
 endmodule
 
+module peepopt_shiftmul_1 (output y, input [2:0] w);
+assign y = 1'b1 >> (w * (3'b110));
+endmodule
+
 module peepopt_muldiv_0(input [1:0] i, output [1:0] o);
 wire [3:0] t;
 assign t = i * 3;