Add a test for instruction shrinking.

cranelift/filetests/isa/x86/shrink-multiple-uses.cton

@@ -0,0 +1,15 @@
+test shrink
+set opt_level=best
+target x86_64
+
+function %test_multiple_uses(i32 [%rdi]) -> i32 {
+ebb0(v0: i32 [%rdi]):
+[Op1rcmp_ib#7083,%rflags]           v3 = ifcmp_imm v0, 0
+[Op2seti_abcd#490,%rax]             v1 = trueif eq v3
+[RexOp2urm_noflags#4b6,%rax]        v2 = bint.i32 v1
+[Op1brib#70]                        brif eq v3, ebb1
+[Op1ret#c3]                         return v2
+
+ebb1:
+[Op2trap#40b]                       trap user0
+}