[Liftoff] Remove unreachable code

Since f32 and f64 constants are loaded into registers right away, we
never need to spill them as constants later.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I7da91bb995e5127b0a9cb1a12a0fcd6566ed98ff
Reviewed-on: https://chromium-review.googlesource.com/960943Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51915}

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

@@ -379,10 +379,8 @@ void LiftoffAssembler::Spill(uint32_t index, WasmValue value) {
       mov(liftoff::GetHalfStackSlot(2 * index + 1), Immediate(high_word));
       break;
     }
-    case kWasmF32:
-      mov(dst, Immediate(value.to_f32_boxed().get_bits()));
-      break;
     default:
+      // We do not track f32 and f64 constants, hence they are unreachable.
       UNREACHABLE();
   }
 }

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

@@ -328,10 +328,8 @@ void LiftoffAssembler::Spill(uint32_t index, WasmValue value) {
       movl(liftoff::GetHalfStackSlot(2 * index + 1), Immediate(high_word));
       break;
     }
-    case kWasmF32:
-      movl(dst, Immediate(value.to_f32_boxed().get_bits()));
-      break;
     default:
+      // We do not track f32 and f64 constants, hence they are unreachable.
       UNREACHABLE();
   }
 }