[InstCombine] Merge DebugLoc when speculatively hoisting store instruction

Summary: Along with https://reviews.llvm.org/D27804, debug locations need to be merged when hoisting store instructions as well. Not sure if just dropping debug locations would make more sense for this case, but as the branch instruction will have at least different discriminator with the hoisted store instruction, I think there will be no difference in practice.

Reviewers: aprantl, andreadb, danielcdh

Reviewed By: aprantl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29062

llvm-svn: 293372