[DAG] Relax type restriction for store merge

Summary: Allow stores of bitcastable types to be merged by peeking through BITCAST nodes and recasting stored values constant and vector extract nodes as necessary.

Reviewers: jyknight, hfinkel, efriedma, RKSimon, spatel

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D34569

llvm-svn: 310655