cranelift: remove load_complex and store_complex (#3976)

This change removes all variants of `load*_complex` and `store*_complex`
from Cranelift; this is a breaking change to the instructions exposed by
CLIF. The complete list of instructions removed is: `load_complex`,
`store_complex`, `uload8_complex`, `sload8_complex`, `istore8_complex`,
`sload8_complex`, `uload16_complex`, `sload16_complex`,
`istore16_complex`, `uload32_complex`, `sload32_complex`,
`istore32_complex`, `uload8x8_complex`, `sload8x8_complex`,
`sload16x4_complex`, `uload16x4_complex`, `uload32x2_complex`,
`sload32x2_complex`.

The rationale for this removal is that the Cranelift backend now has the
ability to pattern-match multiple upstream additions in order to
calculate the address to access. Previously, this was not possible so
the `*_complex` instructions were needed. Over time, these instructions
have fallen out of use in this repository, making the additional
overhead of maintaining them a chore.

cranelift/codegen/src/simple_gvn.rs

@@ -24,9 +24,7 @@ fn trivially_unsafe_for_gvn(opcode: Opcode) -> bool {
 /// Test that, if the specified instruction is a load, it doesn't have the `readonly` memflag.
 fn is_load_and_not_readonly(inst_data: &InstructionData) -> bool {
     match *inst_data {
-        InstructionData::Load { flags, .. } | InstructionData::LoadComplex { flags, .. } => {
-            !flags.readonly()
-        }
+        InstructionData::Load { flags, .. } => !flags.readonly(),
         _ => inst_data.opcode().can_load(),
     }
 }