Ignore TargetConstant with an illegal type. These

Ignore TargetConstant with an illegal type.  These
are used for passing huge immediates in inline ASM
from the front-end straight down to the ASM writer.
Of course this is a hack, but it is simple, limited
in scope, works in practice, and is what LegalizeDAG
does.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@53553 91177308-0d34-0410-b5e6-96231b3b80d8