[codegen] Don't simplify an operation if it would result in unnecessary legalization;

Converting something like iadd.i64 on a 32-bits architecture into a
iadd_imm.i64 will result in the instruction being legalized back to an
iadd.i64 later on, creating unnecessary churn.

This commit implements avoid doing so, and changes the target ISA to a
64-bits platform for tests than ran into this, as well as making sure
this won't happen on 32-bits platforms.

cranelift/filetests/filetests/simple_preopt/div_by_const_indirect.clif

@@ -1,5 +1,5 @@
 test simple_preopt
-target i686 baseline
+target x86_64 baseline
 
 ; Cases where the denominator is created by an iconst
 

cranelift/filetests/filetests/simple_preopt/simplify32.clif

@@ -0,0 +1,61 @@
+test simple_preopt
+target i686
+
+;; 32-bits platforms.
+
+function %iadd_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = iadd v0, v1
+    return v2
+}
+; sameln: function %iadd_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = iadd_imm v0, 2
+; nextln:     return v2
+; nextln: }
+
+function %isub_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = isub v0, v1
+    return v2
+}
+; sameln: function %isub_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = iadd_imm v0, -2
+; nextln:     return v2
+; nextln: }
+
+function %icmp_imm(i32) -> i32 {
+ebb0(v0: i32):
+    v1 = iconst.i32 2
+    v2 = icmp slt v0, v1
+    v3 = bint.i32 v2
+    return v3
+}
+; sameln: function %icmp_imm
+; nextln: ebb0(v0: i32):
+; nextln:     v1 = iconst.i32 2
+; nextln:     v2 = icmp_imm slt v0, 2
+; nextln:     v3 = bint.i32 v2
+; nextln:     return v3
+; nextln: }
+
+;; Don't simplify operations that would get illegal because of lack of native
+;; support.
+function %iadd_imm(i64) -> i64 {
+ebb0(v0: i64):
+    v1 = iconst.i64 2
+    v2 = iadd v0, v1
+    return v2
+}
+; sameln: function %iadd_imm
+; nextln: ebb0(v0: i64):
+; nextln:     v1 = iconst.i64 2
+; nextln:     v2 = iadd v0, v1
+; nextln:     return v2
+; nextln: }
+

cranelift/filetests/filetests/simple_preopt/simplify64.clif

@@ -1,5 +1,7 @@
 test simple_preopt
-target i686
+target x86_64
+
+;; 64-bits platforms.
 
 function %iadd_imm(i32) -> i32 {
 ebb0(v0: i32):