[LV] Report multiple reasons for not vectorizing under allowExtraAnalysis

[LV] Report multiple reasons for not vectorizing under allowExtraAnalysis

The default behavior of -Rpass-analysis=loop-vectorizer is to report only the
first reason encountered for not vectorizing, if one is found, at which time the
vectorizer aborts its handling of the loop. This patch allows multiple reasons
for not vectorizing to be identified and reported, at the potential expense of
additional compile-time, under allowExtraAnalysis which can currently be turned
on by Clang's -fsave-optimization-record and opt's -pass-remarks-missed.

Removed from LoopVectorizationLegality::canVectorize() the redundant checking
and reporting if we CantComputeNumberOfIterations, as LAI::canAnalyzeLoop() also
does that. This redundancy is caught by a lit test once multiple reasons are
reported.

Patch initially developed by Dror Barak.

Differential Revision: https://reviews.llvm.org/D33396

llvm-svn: 303613