[ubsan] Skip null checks if they are constant-folded away

The IR builder can constant-fold null checks if the pointer operand
points to a constant. If the "is-non-null" check is folded away to
"true", don't emit the null check + branch.

Testing: check-clang, check-ubsan.

This slightly 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 |            25251 |
  | patched, -O0   |            23925 | (-5.3%)
  -------------------------------------

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@300509 91177308-0d34-0410-b5e6-96231b3b80d8