ms inline asm: Fix {call,jmp} fptr (#73207)

https://reviews.llvm.org/D151863 (2023-05) removed
`BaseReg = BaseReg ? BaseReg : 1` (introduced in commit
175d0aee (2013)) and caused a
regression: ensuring a non-zero `BaseReg` was to treat
`static void (*fptr)(); __asm { call fptr }` as non-`AbsMem`
`AsmOperandClass` and generate `call $0`/`callq *fptr(%rip)` instead of
`call ${0:P}`/`callq *fptr`
(The asm template argument modifier `P` is for call targets, while
no modifier is used by other instructions like `mov rax, fptr`)

This patch reinstates the BaseReg-setting statement but places it inside
`if (IsGlobalLV)` for clarify.

The special case is unfortunate, but we also have special case in
similar places (https://reviews.llvm.org/D149920).

Fix: #73033