[IR] Optimize bitfield layout of Value for MSVC

This should save a pointer of padding from all MSVC Value subclasses.

Recall that MSVC will not pack the following bitfields together:
  unsigned Bits : 29;
  unsigned Flag1 : 1;
  unsigned Flag2 : 1;
  unsigned Flag3 : 1;

Add a static_assert because LLVM developers always trip over this
behavior. This regressed in June.

llvm-svn: 262045