machinst x64: refactor imports to use rustfmt convention

This change is a pure refactoring--no change to functionality. It removes newlines between the `use ...` statements in the x64 backend so that rustfmt can format them according to its convention. I noticed some files had followed a manual convention but subsequent additions did not seem to fit; this change fixes that and lightly coalesces some of the occurrences of `use a::b; use a::c;` into `use::{b, c}`.

cranelift/codegen/src/isa/x64/inst/emit_tests.rs

@@ -9,10 +9,9 @@
 //! (cd cranelift/codegen && \
 //! RUST_BACKTRACE=1 cargo test isa::x64::inst::test_x64_insn_encoding_and_printing -- --nocapture)
 
-use alloc::vec::Vec;
-
 use super::*;
 use crate::isa::test_utils;
+use alloc::vec::Vec;
 
 #[test]
 fn test_x64_emit() {