[PHIElimination] Fix accounting for undef uses when updating LiveVariables

PHI elimination updates LiveVariables info as described here:

    // We only need to update the LiveVariables kill of SrcReg if this was the
    // last PHI use of SrcReg to be lowered on this CFG edge and it is not live
    // out of the predecessor. We can also ignore undef sources.

Unfortunately if the last use also happened to be an undef use then it
would fail to update the LiveVariables at all. Fix this by not counting
undef uses in the VRegPHIUse map.

Thanks to Mikael Holmén for the test case!

Differential Revision: https://reviews.llvm.org/D111552