[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