[LSR] Create SCEVExpander earlier, use member isSafeToExpand() (NFC)

This is a followup to D129630, which switches LSR to the member
isSafeToExpand() variant, and removes the freestanding function.

This is done by creating the SCEVExpander early (already during the
analysis phase). Because the SCEVExpander is now available for the
whole lifetime of LSRInstance, I've also made it into a member
variable, rather than passing it around in even more places.

Differential Revision: https://reviews.llvm.org/D129769