[NFC] Only expose getXXXSize functions in TypeSize

Currently 'TypeSize' exposes two functions that serve the same purpose:
 - getFixedSize / getFixedValue
 - getKnownMinSize / getKnownMinValue

source : https://github.com/llvm/llvm-project/blob/bf82070ea465969e9ae86a31dfcbf94c2a7b4c4c/llvm/include/llvm/Support/TypeSize.h#L337-L338

This patch offers to remove one of the two and stick to a single function in the code base.

Differential Revision: https://reviews.llvm.org/D141134