cranelift: Restrict br_table to i32 indices (#4510)

* cranelift: Restrict `br_table` to `i32` indices

In #4498 it was proposed that we should only accept `i32` indices
to `br_table`. The rationale for this is that larger types lead the
users to a false sense of flexibility (since we don't support jump
tables larger than u32's), and narrower types are not well tested
paths that would be safer if we removed them.

* cranelift: Reduce directly from i128 to i32 in Switch

cranelift/filetests/filetests/isa/aarch64/jumptable.clif

@@ -2,30 +2,30 @@ test compile precise-output
 set unwind_info=false
 target aarch64
 
-function %f(i64) -> i64 {
+function %f(i32) -> i32 {
   jt0 = jump_table [block1, block2, block3]
 
-block0(v0: i64):
+block0(v0: i32):
   br_table v0, block4, jt0
 
 block1:
-  v1 = iconst.i64 1
+  v1 = iconst.i32 1
   jump block5(v1)
 
 block2:
-  v2 = iconst.i64 2
+  v2 = iconst.i32 2
   jump block5(v2)
 
 block3:
-  v3 = iconst.i64 3
+  v3 = iconst.i32 3
   jump block5(v3)
 
 block4:
-  v4 = iconst.i64 4
+  v4 = iconst.i32 4
   jump block5(v4)
 
-block5(v5: i64):
-  v6 = iadd.i64 v0, v5
+block5(v5: i32):
+  v6 = iadd.i32 v0, v5
   return v6
 }
 
@@ -54,6 +54,5 @@ block5(v5: i64):
 ; block8:
 ;   b label9
 ; block9:
-;   add x0, x0, x5
+;   add w0, w0, w5
 ;   ret
-