[llvm] [RISCV][GISEL] Do not initialize GlobalISel objects unless needed (PR #98233)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I collected data on `ninja check-llvm`. The way to reproduce my experiment is as follows:

1. Build LLVM for RISCV target only
2. checkout this patch
3. Run `ninja check-llvm` to make sure everything is built
4. Run `time ninja check-llvm`
5. Repeat step 4 multiple times and take the average
6. checkout the commit before this patch
7. Run `ninja check-llvm` to make sure everything is built
8. Run `time ninja check-llvm`
9. Repeat step 8 multiple times and take the average

Average without patch: 542.65s
Average with patch: 523.43s
This gives me me a roughly 3.54% speedup as a result of this patch.

https://github.com/llvm/llvm-project/pull/98233