[ubsan] Skip alignment checks on allocas with known alignment
It's possible to determine the alignment of an alloca at compile-time.
Use this information to skip emitting some runtime alignment checks.
Testing: check-clang, check-ubsan.
This significantly reduces the amount of alignment checks we emit when
compiling X86ISelLowering.cpp. Here are the numbers from patched/unpatched
clangs based on r301361.
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| Setup | # of alignment checks |
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| unpatched, -O0 | 47195 |
| patched, -O0 | 30876 | (-34.6%)
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llvm-svn: 301377