Assume lane masks are always precise

Allowing imprecise lane masks in case of more than 32 sub register lanes
lead to some tricky corner cases, and I need another bugfix for another
one. Instead I rather declare lane masks as precise and let tablegen
abort if we do not have enough bits.

This does not affect any in-tree target, even AMDGPU only needs 16 lanes
at the moment. If the 32 lanes turn out to be a problem in the future,
then we can easily change the LaneBitmask typedef to uint64_t.

Differential Revision: http://reviews.llvm.org/D14557

llvm-svn: 253279

llvm/lib/CodeGen/VirtRegMap.cpp

@@ -400,14 +400,6 @@ void VirtRegRewriter::rewrite() {
                 MO.setIsUndef(true);
             } else if (!MO.isDead()) {
               assert(MO.isDef());
-              // Things get tricky when we ran out of lane mask bits and
-              // merged multiple lanes into the overflow bit: In this case
-              // our subregister liveness tracking isn't precise and we can't
-              // know what subregister parts are undefined, fall back to the
-              // implicit super-register def then.
-              LaneBitmask LaneMask = TRI->getSubRegIndexLaneMask(SubReg);
-              if (TargetRegisterInfo::isImpreciseLaneMask(LaneMask))
-                SuperDefs.push_back(PhysReg);
             }
           }