[RISCV] Permit larger RVV stacks and stack offsets

This patch teaches the compiler to generate code to handle larger RVV
stack sizes and stack offsets which resolve an amount larger than 2047
vector registers in size.

The previous behaviour was asserting on such large values as it was only
able to materialize the constant by feeding it to the 12-bit immediate
of an `ADDI` instruction. The compiler can now materialize this amount
into a temporary register before continuing with the computation.

A test case for this scenario is included which also checks that the
temporary register used to materialize the amount doesn't require an
additional spill slot over what we're already reserving for RVV code.

Reviewed By: rogfer01

Differential Revision: https://reviews.llvm.org/D104727