Do not use MRI::getMaxLaneMaskForVReg as a mask covering whole register

MRI::getMaxLaneMaskForVReg does not always cover the whole register.
For example, on X86 the upper 16 bits of EAX cannot be accessed via
any subregister. Consequently, there is no lane mask that only covers
that part of EAX. The getMaxLaneMaskForVReg will return the union of
the lane masks for all subregisters, and in case of EAX, that union
will not cover the upper 16 bits.

This fixes https://llvm.org/bugs/show_bug.cgi?id=29132

llvm-svn: 279969