[codegen] reintroduce support for carry and borrow instructions in RI… (#1005)

Reintroduce support for iadd carry variants and isub borrow variants for
RISC ISAs which had been removed in
https://github.com/CraneStation/cranelift/pull/961 and
https://github.com/CraneStation/cranelift/pull/962 because of the lack
of a proper flags register in RISC architectures.

cranelift/codegen/meta/src/isa/x86/legalize.rs

@@ -305,7 +305,7 @@ pub(crate) fn define(shared: &mut SharedDefinitions, x86_instructions: &Instruct
     Use x86-specific instructions if needed."#,
     )
     .isa("x86")
-    .chain_with(shared.transform_groups.by_name("narrow").id);
+    .chain_with(shared.transform_groups.by_name("narrow_flags").id);
 
     // SIMD
     let uimm8_zero = Literal::constant(&imm.uimm8, 0x00);