This relands #141031
This change ensures generated SPIR-V is valid and passes machine
verification:
```
*** Bad machine code: inconsistent constant size ***
- function: foo
- basic block: %bb.1 entry (0x9ec9298)
- instruction: %12:iid(s8) = G_CONSTANT i4 1
```
That is done by promoting `G_CONSTANT` instructions with small integer
types (e.g., `i4`) to `i8` if no extensions for "special" integer types
are enabled.
435d8b12ef