Add a flag vectorizer-maximize-bandwidth in loop vectorizer to enable using...

Add a flag vectorizer-maximize-bandwidth in loop vectorizer to enable using larger vectorization factor.

To be able to maximize the bandwidth during vectorization, this patch provides a new flag vectorizer-maximize-bandwidth. When it is turned on, the vectorizer will determine the vectorization factor (VF) using the smallest instead of widest type in the loop. To avoid increasing register pressure too much, estimates of the register usage for different VFs are calculated so that we only choose a VF when its register usage doesn't exceed the number of available registers.

llvm-svn: 251592