x64: lower i8x16.popcnt to VPOPCNTB when possible

When AVX512VL or AVX512BITALG are available, Wasm SIMD's `popcnt`
instruction can be lowered to a single x64 instruction, `VPOPCNTB`,
instead of 8+ instructions.

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

@@ -3895,6 +3895,12 @@ fn test_x64_emit() {
         "vcvtudq2ps %xmm2, %xmm8",
     ));
 
+    insns.push((
+        Inst::xmm_unary_rm_r_evex(Avx512Opcode::Vpopcntb, RegMem::reg(xmm2), w_xmm8),
+        "62727D0854C2",
+        "vpopcntb %xmm2, %xmm8",
+    ));
+
     // Xmm to int conversions, and conversely.
 
     insns.push((
@@ -4308,6 +4314,7 @@ fn test_x64_emit() {
     isa_flag_builder.enable("has_sse41").unwrap();
     isa_flag_builder.enable("has_avx512f").unwrap();
     isa_flag_builder.enable("has_avx512dq").unwrap();
+    isa_flag_builder.enable("has_avx512vl").unwrap();
     let isa_flags = x64::settings::Flags::new(&flags, isa_flag_builder);
 
     let rru = regs::create_reg_universe_systemv(&flags);