[maglev] Remove UNREACHABLE from constexpr functions

gcc builds seem to have troubles with this:

 error: call to non-constexpr function

Remove the UNREACHABLE() calls for now but keep a TODO to
potentially re-enable them again once we've figured this out.

Bug: v8:7700,v8:12667
Change-Id: I9628a1326c1c4200b90aad25cd2eabfeb06608fb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3494237
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79314}

src/maglev/maglev-regalloc-data.h

@@ -27,7 +27,10 @@ constexpr uint8_t MapRegisterToIndex(Register r) {
   count++;
   ALWAYS_ALLOCATABLE_GENERAL_REGISTERS(EMIT_BRANCH)
 #undef EMIT_BRANCH
-  UNREACHABLE();
+  // TODO(v8:7700): Re-enable UNREACHABLE once we figure out how to to avoid
+  // the gcc error 'call to non-constexpr function'.
+  // UNREACHABLE();
+  return 255;
 }
 
 constexpr Register MapIndexToRegister(int i) {
@@ -37,7 +40,10 @@ constexpr Register MapIndexToRegister(int i) {
   count++;
   ALWAYS_ALLOCATABLE_GENERAL_REGISTERS(EMIT_BRANCH)
 #undef EMIT_BRANCH
-  UNREACHABLE();
+  // TODO(v8:7700): Re-enable UNREACHABLE once we figure out how to to avoid
+  // the gcc error 'call to non-constexpr function'.
+  // UNREACHABLE();
+  return no_reg;
 }
 
 struct RegisterStateFlags {