[LoongArch] Fixing the incorrect return value of LoongArchTTIImpl::getRegisterBitWidth (llvm#79441)

When we do not enable vector features, we should return the default
value (`TargetTransformInfoImplBase::getRegisterBitWidth`) instead of
zero.

This should fix the LoongArch [buildbot
breakage](https://lab.llvm.org/staging/#/builders/5/builds/486) from
llvm#78943.

(cherry picked from commit 1e9924c)

Author: wangleiat

llvm/lib/Target/LoongArch/LoongArchTargetTransformInfo.cpp

@@ -21,17 +21,20 @@ using namespace llvm;
 
 TypeSize LoongArchTTIImpl::getRegisterBitWidth(
     TargetTransformInfo::RegisterKind K) const {
+  TypeSize DefSize = TargetTransformInfoImplBase::getRegisterBitWidth(K);
   switch (K) {
   case TargetTransformInfo::RGK_Scalar:
     return TypeSize::getFixed(ST->is64Bit() ? 64 : 32);
   case TargetTransformInfo::RGK_FixedWidthVector:
-    if (ST->hasExtLASX() && ST->hasExpAutoVec())
+    if (!ST->hasExpAutoVec())
+      return DefSize;
+    if (ST->hasExtLASX())
       return TypeSize::getFixed(256);
-    if (ST->hasExtLSX() && ST->hasExpAutoVec())
+    if (ST->hasExtLSX())
       return TypeSize::getFixed(128);
-    return TypeSize::getFixed(0);
+    [[fallthrough]];
   case TargetTransformInfo::RGK_ScalableVector:
-    return TypeSize::getScalable(0);
+    return DefSize;
   }
 
   llvm_unreachable("Unsupported register kind");