[llvm][Type] Return fixed size for scalar types. [NFC]

Summary:
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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76892