[llvm][ADT] Fix uint64_t array BitSet construction on 32-bit systems (#162814)

When the underlying storage element is 32-bit, we may only need half of
the last value in the uint64_t array. I've adjusted the constructor to
account for that.

For example, if you construct a 65 bit bitset you will need both 32-bit
halves of the first array value but only the bottom half of the second
value. The storage will only have 3 elements, so attempting to assign
the top 32-bits into the storage will fail.

This happened on our buildbot:
https://lab.llvm.org/buildbot/#/builders/154/builds/22555
(though the traceback is not useful)

Then added tests for < 32 bit sizes, and assertions for the number of
elements we decide to use. Given that the only member of BitSet is a
std::array, I think the size will be consistent across systems.

Tested on 32 and 64-bit Arm machines.

Follow up to #162703.

0 files changed, 0 insertions, 0 deletions