Closes https://github.com/llvm/llvm-project/issues/121700

Up to now, the only way to set specified FMF flags in IRBuilder is to
use `FastMathFlagGuard`. It makes the code ugly and hard to maintain.

This patch introduces a helper class `FMFSource` to replace the original
parameter `Instruction *FMFSource` in IRBuilder. To maximize the
compatibility, it accepts an instruction or a specified FMF.
This patch also removes the use of `FastMathFlagGuard` in some simple
cases.

Compile-time impact:
https://llvm-compile-time-tracker.com/compare.php?from=f87a9db8322643ccbc324e317a75b55903129b55&to=9397e712f6010be15ccf62f12740e9b4a67de2f4&stat=instructions%3Au

With the introduction of CmpPredicate in 51a895a (IR: introduce struct
with CmpInst::Predicate and samesign), PatternMatch is one of the first
key pieces of infrastructure that must be updated to match a CmpInst
respecting samesign information. Implement this change to Cmp-matchers.

This is a preparatory step in migrating the codebase over to
CmpPredicate. Since we no functional changes are desired at this stage,
we have chosen not to migrate CmpPredicate::operator==(CmpPredicate)
calls to use CmpPredicate::getMatching(), as that would have visible
impact on tests that are not yet written: instead, we call
CmpPredicate::operator==(Predicate), preserving the old behavior, while
also inserting a few FIXME comments for follow-ups.

if (C1 & C2) == C2 then (X & C1) - (X & C2) --> X & (C1 ^ C2)

Alive2: https://alive2.llvm.org/ce/z/JvQU8w

Make sure we only access cached nowrap flags.

If both the geps and the subs are nuw the new sub is also nuw.

Proof: https://alive2.llvm.org/ce/z/mM8UvF

Alive2: https://alive2.llvm.org/ce/z/oSWe5S
Closes https://github.com/llvm/llvm-project/issues/118155

This matches m_Select(m_Value(), L, R) or m_Select(m_Value(), R, L).

The (extended) bit width might not fit into the (non-extended)
type, resulting in an incorrect truncation of the compared value.

Fix this by using m_SpecificInt(), which is both simpler and
handles this correctly.

Fixes the assertion failure reported in:
https://github.com/llvm/llvm-project/pull/114539#issuecomment-2485799395

This patch re-queue users of phi when one of its incoming add
instructions is updated. If an add instruction is updated, the analysis
results of phis may be improved. Thus we may further fold some users of
this phi node.

See the following case:
```
define i8 @trunc_in_loop_exit_block() {
; CHECK-LABEL: @trunc_in_loop_exit_block(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br label [[LOOP:%.*]]
; CHECK:       loop:
; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ]
; CHECK-NEXT:    [[PHI:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[IV_NEXT]], [[LOOP_LATCH]] ]
; CHECK-NEXT:    [[CMP:%.*]] = icmp samesign ult i32 [[IV]], 100
; CHECK-NEXT:    br i1 [[CMP]], label [[LOOP_LATCH]], label [[EXIT:%.*]]
; CHECK:       loop.latch:
; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
; CHECK-NEXT:    br label [[LOOP]]
; CHECK:       exit:
; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i32 [[PHI]] to i8
; CHECK-NEXT:    ret i8 [[TRUNC]]
;
entry:
  br label %loop

loop:
  %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop.latch ]
  %phi = phi i32 [ 1, %entry ], [ %iv.next, %loop.latch ]
  %cmp = icmp ult i32 %iv, 100
  br i1 %cmp, label %loop.latch, label %exit

loop.latch:
  %iv.next = add i32 %iv, 1
  br label %loop

exit:
  %trunc = trunc i32 %phi to i8
  ret i8 %trunc
}
```
`%iv u< 100` -> infer `nsw/nuw` for `%iv.next = add i32 %iv, 1`
-> `%iv` is non-negative -> infer `samesign` for `%cmp = icmp ult i32
%iv, 100`.
Without re-queuing users of phi nodes, we cannot improve `%cmp` in one
iteration.

Address review comment
https://github.com/llvm/llvm-project/pull/112642#discussion_r1804712271.
This patch also fixes some non-fixpoint issues in tests.