x64: lower iabs.i64x2 using a single AVX512 instruction when possible (#2819)

* x64: add EVEX encoding mechanism

Also, includes an empty stub module for the VEX encoding.

* x64: lower abs.i64x2 to VPABSQ when available

* x64: refactor EVEX encodings to use `EvexInstruction`

This change replaces the `encode_evex` function with a builder-style struct, `EvexInstruction`. This approach clarifies the code, adds documentation, and results in slight speedups when benchmarked.

* x64: rename encoding CodeSink to ByteSink

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

@@ -3865,6 +3865,12 @@ fn test_x64_emit() {
         "cvtdq2pd %xmm2, %xmm8",
     ));
 
+    insns.push((
+        Inst::xmm_unary_rm_r_evex(Avx512Opcode::Vpabsq, RegMem::reg(xmm2), w_xmm8),
+        "6272FD081FC2",
+        "vpabsq  %xmm2, %xmm8",
+    ));
+
     // Xmm to int conversions, and conversely.
 
     insns.push((
@@ -4276,6 +4282,7 @@ fn test_x64_emit() {
     let mut isa_flag_builder = x64::settings::builder();
     isa_flag_builder.enable("has_ssse3").unwrap();
     isa_flag_builder.enable("has_sse41").unwrap();
+    isa_flag_builder.enable("has_avx512f").unwrap();
     let isa_flags = x64::settings::Flags::new(&flags, isa_flag_builder);
 
     let rru = regs::create_reg_universe_systemv(&flags);