[LLVM][IR] Add textual shorthand for specifying constant vector splats. (#74620)

Add LL parsing for `<N x ty> splat(ty <imm>)` that lowers onto
ConstantInt::get() for integer types and ConstantFP::get() for
floating-point types.

The intent is to extend ConstantInt/FP classes to support vector types
rather than redirecting to other constant classes as the get() methods
do today.

This patch gives IR writers the convenience of using the shorthand
today, thus allowing existing tests to be ported.