[llvm][Type] Return fixed size for scalar types. [NFC]
It is safe to assume that the TypeSize associated to scalar types has
a fixed size.
This avoids an implicit cast of TypeSize to integer inside
Type::getScalarSizeInBits(), as such implicit cast is deprecated.
Reviewers: efriedma, sdesmalen
Reviewed By: efriedma
Subscribers: hiraditya, llvm-commits
Differential Revision: https://reviews.llvm.org/D76892