AsmParser: Don't crash on short hex constants for fp128 types

If we see 0xL01, treat it like 0xL00000000000000000000000000000001
instead of crashing.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@223811 91177308-0d34-0410-b5e6-96231b3b80d8