[LEB128] Don't handle edge cases in every loop iteration

Previously the overflow check was done for every byte even though it
is only needed for the case where Shift == 63.

(cherry picked from commit 963b186)

Author: adrian-prantl

llvm/include/llvm/Support/LEB128.h

@@ -142,7 +142,8 @@ inline uint64_t decodeULEB128(const uint8_t *p, unsigned *n = nullptr,
       break;
     }
     uint64_t Slice = *p & 0x7f;
-    if ((Shift >= 64 && Slice != 0) || Slice << Shift >> Shift != Slice) {
+    if (Shift >= 63 && ((Shift == 63 && (Slice << Shift >> Shift) != Slice) ||
+                        (Shift > 63 && Slice != 0))) {
       if (error)
         *error = "uleb128 too big for uint64";
       Value = 0;
@@ -177,8 +178,8 @@ inline int64_t decodeSLEB128(const uint8_t *p, unsigned *n = nullptr,
     }
     Byte = *p;
     uint64_t Slice = Byte & 0x7f;
-    if ((Shift >= 64 && Slice != (Value < 0 ? 0x7f : 0x00)) ||
-        (Shift == 63 && Slice != 0 && Slice != 0x7f)) {
+    if ((Shift >= 63) && ((Shift == 63 && Slice != 0 && Slice != 0x7f) ||
+                          (Shift > 63 && Slice != (Value < 0 ? 0x7f : 0x00)))) {
       if (error)
         *error = "sleb128 too big for int64";
       if (n)