Assignments with if statements are not optimized

[m-torhan]

Describe the bug
During working on another issue (#1461), we noticed that if statements used to assign values based on some condition are not optimized.

To Reproduce

1. Cherry pick the commit antmicro@3507f0a (with slow_if and fast_if samples).
2. Run IR benchmark for both implementations.
3. Observe critical paths:

• slow_if

Critical path delay: 221ps
Critical path entry count: 2
Critical path:
      0ps as synthesized (+0ps); 0.00%
    221ps (+221ps): sel.10: bits[1024] = sel(cond, cases=[arg2, arg1], id=10, pos=[(0,2,4)])
      0ps (+  0ps): arg1: bits[1024] = param(arg1, id=2)

• fast_if

Critical path delay: 63ps
Critical path entry count: 4
Critical path:
      0ps as synthesized (+0ps); 0.00%
     63ps (+ 19ps): or.27: bits[1024] = or(and.25: bits[1024], nor.26: bits[1024], id=27, pos=[(0,8,18)])
     44ps (+ 27ps): nor.26: bits[1024] = nor(not.24: bits[1024], mask: bits[1024], id=26, pos=[(0,8,26)])
     17ps (+ 17ps): mask: bits[1024] = sign_ext(cond: bits[1], new_bit_count=1024, id=23, pos=[(0,7,15)])
      0ps (+  0ps): cond: bits[1] = param(cond, id=1)

Expected behavior
The slow_if, which uses if statement, should be converted to the equivalent of the fast_if.

Environment (this can be helpful for troubleshooting):

• OS: Arch Linux x86_64
• Kernel version: 6.8.7-arch1-1
• XLS hash: 769ae4e (+ cherry picked antmicro@3507f0a)