[LV] Forced vectorization with runtime checks and OptForSize

[LV] Forced vectorization with runtime checks and OptForSize

When vectorisation is forced with a pragma, we optimise for min size, and we
need to emit runtime memory checks, then allow this code growth and don't run
in an assert like we currently do.

This is the result of D65197 and D66803, and was a use-case not really
considered before. If this now happens, we emit an optimisation remark warning
about the code-size expansion, which can be avoided by not forcing
vectorisation or possibly source-code modifications.

Differential Revision: https://reviews.llvm.org/D67764

llvm-svn: 372694