Removes uint8_t from MachineRepresentation and MachineSemantic enums.

This works around a compiler bug that leads to incorrect masking of
the semantic_ field in TruncatingUseInfoFromRepresentation.

Patch from bulach@google.com

BUG=

Review-Url: https://codereview.chromium.org/2669113003
Cr-Original-Commit-Position: refs/heads/master@{#42925}
Committed: https://chromium.googlesource.com/v8/v8/+/8c7fc377fd5c03e30cbf767cd22aba59178e0143
Review-Url: https://codereview.chromium.org/2669113003
Cr-Commit-Position: refs/heads/master@{#43099}

src/machine-type.h

@@ -15,7 +15,7 @@
 namespace v8 {
 namespace internal {
 
-enum class MachineRepresentation : uint8_t {
+enum class MachineRepresentation {
   kNone,
   kBit,
   kWord8,
@@ -39,7 +39,7 @@ static_assert(static_cast<int>(MachineRepresentation::kLastRepresentation) <
 
 const char* MachineReprToString(MachineRepresentation);
 
-enum class MachineSemantic : uint8_t {
+enum class MachineSemantic {
   kNone,
   kBool,
   kInt32,