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%)
-------------------------------------
llvm-svn: 300509
dbbdda4d23