Merge raw_bitcast and bitcast (#5175)

- Allow bitcast for vectors with differing lane widths
- Remove raw_bitcast IR instruction
- Change all users of raw_bitcast to bitcast
- Implement support for no-op bitcast cases across backends

This implements the second step of the plan outlined here:
https://github.com/bytecodealliance/wasmtime/issues/4566#issuecomment-1234819394

cranelift/codegen/src/isa/s390x/lower.isle

@@ -1760,16 +1760,25 @@
 (rule (lower (has_type $I32 (bitcast x @ (value_type $F32))))
       (vec_extract_lane $F32X4 x 0 (zero_reg)))
 
+;; Bitcast between types residing in GPRs is a no-op.
+(rule 1 (lower (has_type (gpr32_ty _)
+                         (bitcast x @ (value_type (gpr32_ty _))))) x)
+(rule 2 (lower (has_type (gpr64_ty _)
+                         (bitcast x @ (value_type (gpr64_ty _))))) x)
 
-;;;; Rules for `raw_bitcast` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Bitcast between types residing in FPRs is a no-op.
+(rule 3 (lower (has_type (ty_scalar_float _)
+                         (bitcast x @ (value_type (ty_scalar_float _))))) x)
 
-;; FIXME: There are two flavors of raw_bitcast, which are currently not
+;; Bitcast between types residing in VRs is a no-op.
+;; FIXME: There are two flavors of vector bitcast, which are currently not
 ;; distinguished in CLIF IR.  Those generated by Wasmtime assume little-endian
 ;; lane order, and those generated elsewhere assume big-endian lane order.
-;; Raw bitcast is a no-op if current lane order matches that assumed lane order.
+;; Bitcast is a no-op if current lane order matches that assumed lane order.
 ;; However, due to our choice of lane order depending on the current function
 ;; ABI, every bitcast we currently see here is indeed a no-op.
-(rule (lower (raw_bitcast x)) x)
+(rule 4 (lower (has_type (vr128_ty _)
+                         (bitcast x @ (value_type (vr128_ty _))))) x)
 
 
 ;;;; Rules for `insertlane` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

cranelift/codegen/src/isa/s390x/lower.rs

@@ -141,7 +141,6 @@ impl LowerBackend for S390xBackend {
             | Opcode::ScalarToVector
             | Opcode::VhighBits
             | Opcode::Bitcast
-            | Opcode::RawBitcast
             | Opcode::Load
             | Opcode::Uload8
             | Opcode::Sload8