InstrProf: Fix reading of consecutive 32 bit coverage maps

InstrProf: Fix reading of consecutive 32 bit coverage maps

When we generate coverage data, we explicitly set each coverage map's
alignment to 8 (See InstrProfiling::lowerCoverageData), but when we
read the coverage data, we assume consecutive maps are exactly
adjacent. When we're dealing with 32 bit, maps can end on a 4 byte
boundary, causing us to think the padding is part of the next record.

Fix this by adjusting the buffer to an appropriately aligned address
between records.

This is pretty awkward to test, as it requires a binary with multiple
coverage maps to hit, so we'd need to check in multiple source files
and a binary blob as inputs.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239129 91177308-0d34-0410-b5e6-96231b3b80d8