Reland [AArch64][NFC] Call the API getVScaleRange directly

Use the maximum 64 for BitWidth of getVScaleRange to avoid returning an empty range.

the previous changes bring in a Buildbot failure because MinSVEVectorSize = MinSVEVectorSize.
    error: explicitly assigning value of variable of type 'unsigned int' to itself [-Werror,-Wself-assign]

Reviewed By: sdesmalen, nikic, dmgreen
Differential Revision: https://reviews.llvm.org/D155708