[LSR] Combine unfolded offset into invariant register

LSR reassociates constants as unfolded offsets when the constants fit as
immediate add operands, which currently prevents such constants from being
combined later with loop invariant registers.
This patch modifies GenerateCombinations() to generate a second formula which
includes the unfolded offset in the combined loop-invariant register.

This commit fixes a bug in the original patch (committed at r345114, reverted
at r345123).

Differential Revision: https://reviews.llvm.org/D51861

llvm-svn: 346390