Add TRI::getSubRegIndexLaneMask().

Sub-register lane masks are bitmasks that can be used to determine if
two sub-registers of a virtual register will overlap. For example, ARM's
ssub0 and ssub1 sub-register indices don't overlap each other, but both
overlap dsub0 and qsub0.

The lane masks will be accurate on most targets, but on targets that use
sub-register indexes in an irregular way, the masks may conservatively
report that two sub-register indices overlap when the eventually
allocated physregs don't.

Irregular register banks also mean that the bits in a lane mask can't be
mapped onto register units, but the concept is similar.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@163630 91177308-0d34-0410-b5e6-96231b3b80d8

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