[LangRef] Remove incorrect vector alignment rules

The LangRef incorrectly says that if no exact match is found when
seeking alignment for a vector type, the largest vector type smaller
than the sought-after vector type. This is incorrect as vector types
require an exact match, else they fall back to reporting the natural
alignment.

The corrected rule was not added in its place, as rules for other types
(e.g., floating-point types) aren't documented.

A unit test was added to demonstrate this.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D112463