[SPIRV] Promote scalar arguments to vector for `OpExtInst` in `generateExtInst` instead of `SPIRVRegularizer` (#170155)

This patch consist of 2 parts:
* A first part that removes the scalar to vector promotion for built-ins
in the `SPIRVRegularizer`;
* and a second part that implements the promotion for built-ins from
scalar to vector in `generateExtInst`.

The implementation in `SPIRVRegularizer` had several issues:
* It rolled its own built-in pattern matching that was extremely
permissive
  * the compiler would crash if the built-in had a definition
  * the compiler would crash if the built-in had no arguments
* The compiler would crash if there were more than 2 function
definitions in the module.
* It'd be better if this was implemented as a module pass; where we
iterate over the users of the function, instead of scanning the whole
module for callers.

This patch does the scalar to vector promotion just before the
`OpExtInst` is generated. Without relying on the IR transformation.

One change in the generated code from the previous implementation is
that this version uses a single `OpCompositeConstruct` operation to
convert the scalar into a vector. The old implementation inserted an
element at the 0 position in an `undef` vector (using
`OpCompositeInsert`); then copied that element for every vector element
using `OpVectorShuffle`.

This patch also adds a test (`OpExtInst_vector_promotion_bug.ll`) that
highlights an issue in the builtin pattern matching that we're using:
our pattern matching doesn't consider the number of arguments, only the
demangled name, first and last arguments (`min(int,int,int)` matches the same builtin as `min(int, int)`).