[liftoff] [cleanup] Construct RegLists using symbolic names

Now that https://crrev.com/c/728026 has landed, we can construct the
constexpr RegLists using symbolic register names instead of hard-coding
the register codes.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I21e46aeb5e8598a56f641341bcd7cf718fe4fbf9
Reviewed-on: https://chromium-review.googlesource.com/735548Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48906}

src/wasm/baseline/ia32/liftoff-assembler-ia32-defs.h

@@ -13,12 +13,8 @@ namespace wasm {
 
 static constexpr bool kLiftoffAssemblerImplementedOnThisPlatform = true;
 
-static constexpr RegList kLiftoffAssemblerGpCacheRegs = 1 << 0 |  // eax
-                                                        1 << 1 |  // ecx
-                                                        1 << 2 |  // edx
-                                                        1 << 3 |  // ebx
-                                                        1 << 6 |  // esi
-                                                        1 << 7;   // edi
+static constexpr RegList kLiftoffAssemblerGpCacheRegs =
+    Register::ListOf<eax, ecx, edx, ebx, esi, edi>();
 
 }  // namespace wasm
 }  // namespace internal

src/wasm/baseline/x64/liftoff-assembler-x64-defs.h

@@ -13,12 +13,8 @@ namespace wasm {
 
 static constexpr bool kLiftoffAssemblerImplementedOnThisPlatform = true;
 
-static constexpr RegList kLiftoffAssemblerGpCacheRegs = 1 << 0 |  // rax
-                                                        1 << 1 |  // rcx
-                                                        1 << 2 |  // rdx
-                                                        1 << 3 |  // rbx
-                                                        1 << 6 |  // rsi
-                                                        1 << 7;   // rdi
+static constexpr RegList kLiftoffAssemblerGpCacheRegs =
+    Register::ListOf<rax, rcx, rdx, rbx, rsi, rdi>();
 
 }  // namespace wasm
 }  // namespace internal