[ADT][SmallVector] Flip an assert comparison to avoid overflows yielding false-negatives. NFC.

r221973 changed SmallVector::operator[] to use size_t instead of unsigned.

Before that, on 64bit platforms, when a large index (say -1) was passed,
truncating it to unsigned avoided an overflow when computing 'begin() + idx',
and failed the range checking assertion, as expected.
With r221973, idx isn't truncated, so the addition wraps to
'(char*)begin() - 1', and doesn't fire anymore when it should have done so.

This commit changes the comparison to instead compute 'end() - begin()'
(i.e., 'size()'), which avoids potentially overflowing additions, and
correctly triggers the assertion when values such as -1 are passed.
Note that the problem already existed before that revision, on platforms
where sizeof(size_t) == sizeof(unsigned).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225338 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/SmallVector.h

@@ -135,11 +135,11 @@ public:
   const_pointer data() const { return const_pointer(begin()); }
 
   reference operator[](size_type idx) {
-    assert(begin() + idx < end());
+    assert(idx < size());
     return begin()[idx];
   }
   const_reference operator[](size_type idx) const {
-    assert(begin() + idx < end());
+    assert(idx < size());
     return begin()[idx];
   }