ValueMapper: Always use Mapper::mapValue from remapInstruction, NFCI

Use Mapper::mapValue instead of llvm::MapValue from
Mapper::remapInstruction when mapping an incoming block for a PHINode
(follow-up to r265832).  This will implicitly pass along the
Materializer argument, but when this code was added in r133513 there was
no Materializer argument.  I suspect this call to MapValue was just
missed in r182776 since it's not observable (basic blocks can't be
materialized, and they don't reference other values).

llvm-svn: 265833