[RISCV] Add the GlobalMerge pass (disabled by default)

[RISCV] Add the GlobalMerge pass (disabled by default)

Split out from D129178, this just adds the GlobalMerge tests (other than global-merge-minsize.ll which is testing a specific configuration of the pass when it's enabled) and exposes `-riscv-enable-global-merge` and //doesn't enable it by default//.

Note that the comment "// FIXME: Unify control over GlobalMerge." is copied from the Arm and AArch64 backends, which expose the same flag. Presumably the author is imagining some later refactoring that provides a target-independent flag.

Reviewed By: craig.topper, reames, hiraditya

Differential Revision: https://reviews.llvm.org/D130481