Document the maximum LLVM IR alignment, which is 1 << 29 or 0.5 GiB

Add verifier checks.  We already check these in the assembly parser, but
a frontend producing IR in memory wouldn't hit those checks.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213027 91177308-0d34-0410-b5e6-96231b3b80d8