[BasicTTI] getInterleavedMemoryOpCost(): discount unused members of mask if...

[BasicTTI] getInterleavedMemoryOpCost(): discount unused members of mask if mask for gap will be used

As it can be seen in `InnerLoopVectorizer::vectorizeInterleaveGroup()`,
in some cases (reported by `UseMaskForGaps`), the gaps in the interleaved load/store group
will be masked away by another constant mask, so there is no need to
account for the cost of replication of the mask for these.

Differential Revision: https://reviews.llvm.org/D112877