[DAGCombiner] handle more store value forwarding

When lowering calls on target like PPC, some stack loads
will be generated for by value parameters. Node CALLSEQ_START
prevents such loads from being combined.

Suggested by @RolandF, this patch removes the unnecessary
loads for the byval parameter by extending ForwardStoreValueToDirectLoad

Reviewed By: nemanjai, RolandF

Differential Revision: https://reviews.llvm.org/D138899