Fix an assert in SelectionDAGBuilder when processing inline asm

When processing inline asm that contains errors, make sure we can recover
gracefully by creating an UNDEF SDValue for the inline asm statement before
returning from SelectionDAGBuilder::visitInlineAsm. This is necessary for
consumers that don't exit on the first error that is emitted (e.g. clang)
and that would assert later on.

Fixes PR24071.

Patch by Diana Picus.

llvm-svn: 269811