[ubsan] Skip null checks on pointers to the start of an alloca

Pointers to the start of an alloca are non-null, so we don't need to
emit runtime null checks for them.

Testing: check-clang, check-ubsan.

This significantly reduces the amount of null checks we emit when
compiling X86ISelLowering.cpp. Here are the numbers from patched /
unpatched clangs based on r300371.

  | Setup          | # of null checks |
  | unpatched, -O0 |            45439 |
  | patched, -O0   |            25251 | (-44.4%)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300508 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit ae2d1b7d8c6cc3f59ee8f0bdff7757ae4e215ed7)
2 files changed