[Liftoff] Use better value for "undefined Register"

There is no "no_reg" LiftoffRegister, so we sometimes have to
initialize it to an arbitrary value, because it will be overwritten
later anyway. One solution would be to wrap it in base::Optional, but
that does not really reflect the right semantics, since it's not
optional, but just set at a later point.
So far, we used the register with code 0, but on some platforms (e.g.
mips), register 0 is not a Liftoff cache register. So just use the
first gp cache register instead.

R=ahaas@chromium.org

Bug: chromium:883280
Change-Id: I157384ad5a9ca6a20aa198f3217a7d754dc53e30
Reviewed-on: https://chromium-review.googlesource.com/1221216
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55835}

src/wasm/baseline/liftoff-assembler.cc

@@ -286,7 +286,7 @@ void LiftoffAssembler::CacheState::InitMerge(const CacheState& source,
       auto& dst = stack_state[dst_idx];
       auto& src = source.stack_state[src_idx];
       // Just initialize to any register; will be overwritten before use.
-      LiftoffRegister reg(Register::from_code<0>());
+      LiftoffRegister reg = kGpCacheRegList.GetFirstRegSet();
       RegClass rc = src.is_reg() ? src.reg_class() : reg_class_for(src.type());
       if (src.is_reg() && is_free(src.reg())) {
         reg = src.reg();

src/wasm/baseline/liftoff-compiler.cc

@@ -266,14 +266,14 @@ class LiftoffCompiler {
   uint32_t ProcessParameter(ValueType type, uint32_t input_idx) {
     const int num_lowered_params = 1 + needs_reg_pair(type);
     // Initialize to anything, will be set in the loop and used afterwards.
-    LiftoffRegister reg = LiftoffRegister::from_code(kGpReg, 0);
+    LiftoffRegister reg = kGpCacheRegList.GetFirstRegSet();
     RegClass rc = num_lowered_params == 1 ? reg_class_for(type) : kGpReg;
     LiftoffRegList pinned;
     for (int pair_idx = 0; pair_idx < num_lowered_params; ++pair_idx) {
       compiler::LinkageLocation param_loc =
           descriptor_->GetInputLocation(input_idx + pair_idx);
       // Initialize to anything, will be set in both arms of the if.
-      LiftoffRegister in_reg = LiftoffRegister::from_code(kGpReg, 0);
+      LiftoffRegister in_reg = kGpCacheRegList.GetFirstRegSet();
       if (param_loc.IsRegister()) {
         DCHECK(!param_loc.IsAnyRegister());
         int reg_code = param_loc.AsRegister();
@@ -357,7 +357,7 @@ class LiftoffCompiler {
     }
     DCHECK_EQ(input_idx, descriptor_->InputCount());
     // Set to a gp register, to mark this uninitialized.
-    LiftoffRegister zero_double_reg(Register::from_code<0>());
+    LiftoffRegister zero_double_reg = kGpCacheRegList.GetFirstRegSet();
     DCHECK(zero_double_reg.is_gp());
     for (uint32_t param_idx = num_params; param_idx < __ num_locals();
          ++param_idx) {