Move insertlane to ISLE (#3544)

This also fixes a bug where `movsd` was incorrectly used with a memory
operand for `insertlane`, causing it to actually zero the upper bits
instead of preserving them.

Note that the insertlane logic still exists in `lower.rs` because it's
used as a helper for a few other instruction lowerings which aren't
migrated to ISLE yet. This commit also adds a helper in ISLE itself for
those other lowerings to use when they get implemented.

Closes #3216

cranelift/codegen/src/prelude.isle

@@ -166,6 +166,10 @@
 (decl writable_reg_to_reg (WritableReg) Reg)
 (extern constructor writable_reg_to_reg writable_reg_to_reg)
 
+;; Extract a `u8` from an `Uimm8`.
+(decl u8_from_uimm8 (u8) Uimm8)
+(extern extractor infallible u8_from_uimm8 u8_from_uimm8)
+
 ;; Extract a `u64` from an `Imm64`.
 (decl u64_from_imm64 (u64) Imm64)
 (extern extractor infallible u64_from_imm64 u64_from_imm64)