[AsmPrinter] Fix bit pattern for i1 vectors.

Vectors are defined to be tightly packed, regardless of the element
type. The AsmPrinter didn't realize this, and was allocating extra
padding.

Fixes https://github.com/llvm/llvm-project/issues/49286
Fixes https://github.com/llvm/llvm-project/issues/53246
Fixes https://github.com/llvm/llvm-project/issues/55522

Differential Revision: https://reviews.llvm.org/D129164