[CanonicalizeInst] Remove redundant lexical bbis.
[CanonicalizeInst] Remove redundant lexical bbis.

Previously, CanonicalizeInstruction::eliminateSimpleBorrows bailed out
when encountering any any [lexical] begin_borrow.  While generally such
borrow scopes must be preserved, they may be removed if they are
redundant (when the borrowed value is guaranteed by another means: an
outer lexical borrow scope or a guaranteed function argument).  Here,
removing such redundant lexical borrow scopes is enabled.

apple/swift,atrick/swift,benlangmuir/swift,atrick/swift,ahoppen/swift,JGiola/swift,glessard/swift,benlangmuir/swift,roambotics/swift,gregomni/swift,gregomni/swift,rudkx/swift,glessard/swift,roambotics/swift,roambotics/swift,glessard/swift,rudkx/swift,atrick/swift,benlangmuir/swift,rudkx/swift,ahoppen/swift,JGiola/swift,apple/swift,glessard/swift,apple/swift,atrick/swift,benlangmuir/swift,gregomni/swift,apple/swift,rudkx/swift,roambotics/swift,ahoppen/swift,apple/swift,gregomni/swift,gregomni/swift,atrick/swift,JGiola/swift,roambotics/swift,ahoppen/swift,glessard/swift,ahoppen/swift,glessard/swift,rudkx/swift,atrick/swift,ahoppen/swift,rudkx/swift,JGiola/swift,benlangmuir/swift,benlangmuir/swift,JGiola/swift,gregomni/swift,roambotics/swift,JGiola/swift,apple/swift