[RegisterCoalescer] Don't set read-undef in pruneValues, only clear

Summary:
The comments in the code said

 // Remove <def,read-undef> flags. This def is now a partial redef.

but the code didn't just remove read-undef, it could introduce new ones which
could cause errors.

E.g. if we have something like

%vreg1<def> = IMPLICIT_DEF
%vreg2:subreg1<def, read-undef> = op %vreg3, %vreg4
%vreg2:subreg2<def> = op %vreg6, %vreg7

and we merge %vreg1 and %vreg2 then we should not set undef on the second subreg
def, which the old code did.

Now we solve this by actually do what the code comment says. We remove
read-undef flags rather than remove or introduce them.

Reviewers: qcolombet, MatzeB

Reviewed By: MatzeB

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D38616

llvm-svn: 315564