Re-commit r301040 "X86: Don't emit zero-byte functions on Windows"

Re-commit r301040 "X86: Don't emit zero-byte functions on Windows"

In addition to the original commit, tighten the condition for when to
pad empty functions to COFF Windows.  This avoids running into problems
when targeting e.g. Win32 AMDGPU, which caused test failures when this
was committed initially.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301047 91177308-0d34-0410-b5e6-96231b3b80d8