[scudo] Minor code generation improvement

Summary:
It looks like clang was generating somewhat weird assembly with the current
code. `FromPrimary`, even though `const`,  was replaced every time with the code
generated for `size <= SizeClassMap::kMaxSize` instead of using a variable or
register, and `FromPrimary` didn't induce `ClassId != 0` for the compiler, so a
dead branch was generated for `getActuallyAllocatedSize(Ptr, ClassId)` since
it's never called for `ClassId = 0` (Secondary backed allocations) [this one
was more wishful thinking on my side than anything else].

I rearranged the code bit so that the generated assembly is less clunky.

Also changed 2 whitespace inconsistencies that were bothering me.

Reviewers: alekseyshl, flowerhack

Reviewed By: flowerhack

Subscribers: llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D40976

llvm-svn: 320160